Kennecott Copper Corp. and Phelps Dodge Corp., the nation's largest copper producers, reported declines in their second-quarter operating profits, due primarily to a decline in copper prices.

Kennecott, the number one producer, reported an operating profit of $3.4 million (10 cent a share) compared with $5.5 million (11 cents) a year ago. Sales came to $478.2 million compared with $279.4 million.

Phelps Dodge said its second-quarter profit dropped to $11 million (45 cents a share) from $17.5 million (85 cents) a year ago. Sales declined to $251.9 million from $274.6 million.

For the six months, Kennecott reported an operating profit of $8.7 million (25 cents) compared with $10.5 million (32 cents) a year ago. Sales rose to $906 million from $512 million.

Lower worldwide demand for crude oil and higher operating costs resulted in a 33.4 percent decline in second-quarter earnings for Texaco Inc. from the same period last year.

Texaco reported earnings of $154.4 million (58 cents a share), a sharp drop from last year's net income of $237.9 million (88 cents).

Included in earnings were foreign currency translation losses of $12.3 million caused primarily by the weakened value of the dollar against the Japanese yen. A year ago, currency exchange losses were $8.9 million.

In the three months, Texaco said revenues totaled $6.83 billion, a drop of 3.7 percent from $7.09 billion last year.

Texaco chairman Maurice Granville said results from refining, marketing and supply activities outside the United States were unsatisfactory.He noted that gains in revenues from higher product prices were offset by inflationary increases in expenses and lower profits from crude oil processing.

He added, however, that second-quarter results from production in the United States showed a slight improvement from last year, a reflection of higher natural gas and crude oil prices.