By Mackenzie Tatananni and Nate Wolf

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 61.6% 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 36.1% 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 33.1% 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 24.8% 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 23.4% 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 nearly 48.9%. 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 overly 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 32.5%. The real estate investment trust swung to a loss in the third quarter and issued full-year guidance that underwhelmed investors. Alexandria lowered the midpoint of its earnings guidance to a loss of $2.94 a share from earnings of 50 cents a share, citing the potential for additional real estate impairments in the fourth quarter.

Molina Healthcare sank 21% in October. 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.

CoStar Group declined 20.7%. CoStar's third-quarter earnings report Tuesday put it squarely in the spotlight. While the real estate data-analytics company issued an upbeat outlook for the full year and posted adjusted earning that beat consensus, CoStar reported a net loss of $30.9 million versus a profit of $53 million last year.

Cybersecurity company F5 declined 20.6%. 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."

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

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October 31, 2025 13:08 ET (17:08 GMT)

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