Geico Corp., the holding company that owns the Government Employes Insurance companies, reported yesterday that its operating income fell about 5 percent last year, to $59.55 million from $62.41 million in 1978.

Geico Chairman John J. Byrne called the performance, "a reasonably prosperous year for Geico" and noted that the company's insurance premium income increased in 1979 after several years of declines.

The insurance company's net income for the year ended Dec. 31 was down by almost 16 percent, to $72.3 million from $88.2 million.

Last year's net income included a $15.5 million tax credit resulting from previous losses and in 1978 Geico counted $32.7 million in tax loss credits. In 1979, Geico wrote off only $724,000 on investments, down sharply from the $6.96 million investment loss in the previous year, when the company unloaded several unprofitable items in its portfolio.

On an earnings per share basis, Geico reported an increase in operating income to $2.14 a share from $1.74 and an increase in net earnings to $2.67 from $2.49. Earnings per share increased when total earnings were declining because of changes in the company's financial structure. During 1979 Geico exchanged about 5.9 million shares of common and preferred stock for debentures, reducing the number of shares outstanding by more than 19 percent.

Insurance premiums for the year increased to $639.3 million from $625.7 million.

For the final quarter of 1979 Geico's operating income was down sharply, to $13.5 million (53 cents) from $18.3 million (51 cents) and net income was off more than 50 percent, to $12.5 million (49 cents) from $26.7 million (75 cents.)

Government Employees Life Insurance Company reported its 1979 profits increased to $8.16 million ($1.82 a share) from $7.99 million ($1.79) the year before.

The Geico Corp. life insurance subsidiary reported premium income for the year increased 20 percent to $35 million and investment income for the year grew to $11.7 million from $10.7 million.

The unaudited results showed earnings for the fourth quarter of $2.25 million (50 cents), up from $1.83 million (41 cents) in the same period a year ago.

Logetronics Inc. of Springfield posted a 6 percent increase in operating income last year, but its net earnings were down slightly from the previous year's, which included a large extraordinary credit.

The company reported income for the 12 months ended Dec. 31 of $1.24 million ($1.17 a share), up from 1978 operating earnings of $1.18 million ($1.15). A special credit of $110,000 (11 cents) boosted total 1978 earnings to $1.28 million ($1.26.)

Logetronics said revenues for the year climbed 30 percent to $28.38 million from $21.91 million.

Logetronics' performance for the year was aided by a strong fourth quarter in which profits jumped 44 percent to $404,000 (37 cents) from $247,000 (24 cents), and revenues climbed to $8.1 million from $5.6 million.

Potomac Electric Power Co. reported common stock earnings for the 12 months through January increased to $74 million ($1.79 a share) from $66.9 million ($1.66) in the same period ending a year ago.

Pepco's revenues for the period climbed to $754 million from $717 million and net income increased to $87.4 million from $77.3 million.

For the month of January, Pepco posted revenues of $59.7 million, net income of $6.9 million, and earnings for common stock of $5.6 million (13 cents.)

Baltimore Gas and Electric said its common stock earnings for the 12 months ending in January fell slightly, to $102.8 million ($3.27 a share) from $103.6 million ($3.36.)

The Maryland utility's revenues for the period increased to just over $1 billion from $985 million in the year ended in January 1979 and net income increased to $122.9 million from $121.8 million.

For the month of January, Baltimore Gas reported operating revenue of $96.3 million, net income of $6.7 million and common stock earnings of $4.9 million (15 cents a share.)

The Federal Home Loan Mortgage Corp., known as Freddie Mac, reported its net income last year increased to $35.6 million from $25.4 million in 1978. i

The federally chartered company, which provides funds for home mortgages by purchasing mortgages from lenders, bought $5.7 billion worth of mortgages and sold $4.5 billion in securities to obtain funds.

At year end, Freddie Mac held mortgages worth $19.4 billion, up from $15.1 billion a year earlier. Most of the increase in profits in 1979 was because of unexpected profits on short-term investments, made possible by current high interest rates, the corporation said.

Tech Serv Inc. of Beltsville turned in an operating loss of $77,600 (14 cents a share) for the six months ended Dec. 31, compared with a profit of $22,300 (4 cents) in the same period of 1978.

Sale of some property owned by the company reduced the total loss to $29.200 (5 cents), the company said.

Sales for the six months increased to $645,000 from $559,000 the year before. Tech Serv, which makes radio-controlled targets for the armed services, said a new order from the Army in January should improve its profit picture for the second half of the year.