Exxon Corp., the world's largest company, said yesterday its fourth-quarter after-tax profits rose 26.4 percent, while earnings for all of 1985 fell 11.9 percent.

Exxon said it netted $1.805 billion ($2.43 a share) in the last three months of 1985 compared with a $1.428 billion ($1.81) in the same period the previous year. Revenue edged up to $24.680 billion from $24.557 billion.

For the full year, Exxon's profits totaled $4.870 billion ($6.46), which was down from $5.528 billion ($6.77) in 1984. Revenue fell to $93.211 billion from $97.288 billion.

Commenting on the yearly results, Exxon Chairman C. C. Garvin said stronger earnings in domestic and foreign petroleum and natural gas operations offset lower chemicals earnings and higher corporate and financing costs.

*Chevron Corp. yesterday reported flat net income for 1985, citing declining crude oil prices and sluggish demand for the results.

Net income for the year was $1.547 billion, which is an increase of 1 percent from the $1.534 billion earned in 1984.

The company said a 38 percent increase in earnings in the fourth quarter to $601 million from $434 million in 1984 was responsible for the yearly increase.

A company spokesman said fourth-quarter operating earnings were basically unchanged from 1984, and he attributed the 38 percent rise in net income largely to asset sales connected with Chevron's acquisition of Gulf Corp.

Fourth-quarter earnings per share were $1.75, up from $1.26 for the comparable period in 1984. That boosted yearly per-share earnings to $4.52 from $4.48 in 1984.

The company said 1985 revenue was $45.3 billion, with $10.7 billion in the fourth quarter. For the same two periods in 1984, revenue was $47.4 billion and $13.3 billion, respectively.

*American Telephone & Telegraph Co. said yesterday its net income fell 1.6 percent in the last three months of 1985 from a year earlier, the first such decline AT&T has reported since the Bell System breakup two years ago.

Financial analysts had expected the nation's biggest telephone company to report substantially higher profits. They said the earnings decline apparently came from the high cost of introducing new products that eventually will prove profitable.

AT&T revenue in the quarter rose 8.4 percent to $9.12 billion, the highest since the breakup.

Investors seemed to take the bad news in stride. AT&T stock was down 12 1/2 cents to $22.87 1/2 cents late yesterday afternoon on the New York Stock Exchange.

AT&T said it earned $364 million in the fourth quarter of 1985, down 1.6 percent from $370 million a year earlier. Its earnings per share slipped a penny to 32 cents.

For all of 1985, AT&T said it earned $1.56 billion, up 13.6 percent from $1.37 billion in 1984. Earnings per share rose to $1.37 from $1.25 in 1984. AT&T said its revenue for all of 1985 was $34.91 billion, up 5.2 percent from $33.19 billion in 1984.

*U.S. Steel Corp., the nation's largest steel maker, said yesterday that lower prices for steel products caused its earnings to fall to $409 million in 1985 from $493 million in 1984.

The company, the 15th largest in the United States, said a "continued strong performance" by its Marathon Oil subsidiary compensated somewhat for the weak results in most of its other industry segments. The company's oil and gas segment had operating income before foreign income taxes of $1.27 billion, the same as last year. Overall corporate sales increased to $19.28 billion from $19.10 billion.

For the fourth quarter, the company earned $57 million compared with $29 million last year. Fourth-quarter sales were $4.73 billion compared with $4.66 billion.