These Were the S&P 500’s Best and Worst Stocks in October

Four of the five best performing stocks in the S&P 500 for October were artificial-intelligence plays, including Advanced Micro Devices. (STR/AFP via Getty Images)

Key Points

• Advanced Micro Devices led S&P 500 performers in October.
• Fiserv was the worst performer, with shares cratering due to a slashed full-year outlook and missed earnings.
• Four of the top five S&P 500 stocks in October were AI-related, despite concerns about an “AI bubble.”

October is winding to a close, capping off a month filled with rapidly transforming trade policy and a barrage of earnings from some of the biggest names in tech.

And while concerns have intensified around the formation of a possible “AI bubble,” four of the five best-performing stocks in the benchmark S&P 500 this month were artificial-intelligence plays.

Advanced Micro Devices was by far the top performer in the index, climbing 58% in October. The chip maker announced on Oct. 6 that it had notched a deal to become a major supplier for OpenAI. The ChatGPT maker plans to deploy 6 gigawatts worth of AMD chips in exchange for up to 160 million shares of AMD stock. The deal propelled AMD shares to their best month since 2001, according to Dow Jones Market Data.

Micron Technology continued its banner year in October, with shares rising 34% for the month. AI has driven a surge in demand for the company’s dynamic random-access memory, or DRAM, products, giving Micron license to raise prices as customers’ inventories thin out. The company got an additional boost this week when South Korean rival SK Hynix forecast a “supercycle” of demand for memory chips.

Shares of Teradyne jumped 32% to an all-time high in October. The maker of test systems for semiconductors and robotics closed last week down for the month. But shares surged after the company beat earnings expectations for the third quarter and issued fourth-quarter guidance that blew past analysts’ estimates, citing robust AI-related demand.

J.B. Hunt Transport Services rebounded from a rough year, rising 26% in October. The freight company has combatted a weak shipping economy by controlling costs. That strategy paid off: Hunt posted third-quarter earnings on Oct. 16 that easily beat analysts’ expectations, driving the stock to its largest single-day gain since 1998. It remains narrowly down in 2025.

Data-storage company Western Digital was the second-best performer in the S&P 500 in the third quarter, but it will have to settle for fifth place in October. The stock rose 25% amid a spike in demand for storage hardware due to cloud and AI investments. Another earnings beat this week adds to Western’s momentum heading into the home stretch of 2025.

Conversely, Fiserv was the worst performer in October as shares cratered about 48%. The stock had its worst day on record Wednesday after Fiserv slashed its full-year outlook, sending shares down 44%. CEO Mike Lyons attributed the guidance cut to excessively upbeat expectations for growth in Argentina and a previous focus on “short-term initiatives” that came at the expense of long-term customer relationships. Third-quarter earnings also missed analysts’ expectations.

Alexandria Real Estate Equities fell 30%. The real estate investment trust swung to a loss in the third quarter and issued full-year guidance that underwhelmed investors. Alexandria now says results will fall in a a range whose midpoint is a loss of $2.94 a share, citing the potential for additional real estate impairments in the fourth quarter. It had earlier predicted earnings of 50 cents a share

Cybersecurity company F5 declined 22%. Shares sank by double digits on Oct. 16 alone, when F5 disclosed it had suffered a security breach that gave nation-state affiliated hackers “persistent access” to some of its systems. The Cybersecurity & Infrastructure Security Agency said at the time that the hack presented “an imminent threat to federal networks.”

Shares of potash and phosphate miner Mosaic fell 21% in October, bringing the stock into the red for the year. The company reported preliminary third-quarter phosphate production volumes that fell below management’s expectations. At least eight analysts lowered their price targets for the stock during October, according to FactSet.

Molina Healthcare sank 20%. Last week, the health insurer reduced its full-year earnings guidance for the third time in months, citing cost pressures and underperformance in its marketplace business. Earnings and revenue for the third quarter also came in sharply below Wall Street’s estimates.

Write to Mackenzie Tatananni at mackenzie.tatananni@barrons.com and Nate Wolf at nate.wolf@barrons.com