Riggs National Bank reported yesterday a 27 1/2 percent increase in profitability during the first six months of 1978.

Reflecting generally sharp earnings gains by many of the nation's regional banking institutions this year, Riggs was joined in posting significant gains by National Savings & Trust Co., Citizens Bank & Trust Co. in Maryland, and two Financial General banks - Union First of Washington and American Bank of Maryland.

At Riggs, the largest commercial bank in metropolitan Washington, operating profits for the recent six months totaled $8.55 million ($2.82 a share) compared with $6.7 million ($2.21) in the same period last year. Second-quarter operating profits rose 22 percent to $4.26 million from $3.5 million.

Chairman Vincent Burke Jr. said the higher profits reflected a higher volume of earning assets, accompanied by higher interest rates. Loan volume rose 17 percent to a record $1 billion on June 30 while deposits climbed 14 percent to a record $1.83 billion and assets increased 18 percent to $2.18 billion, also a record.

Included in the Riggs results are operations of a subsidiary, Central Charge Service, the largest regional credit and business in this area.

National Savings & Trust Co., of Washington, reported yesterday an increase of 33 percent in first-half profits to $1.9 million ($4.13 a share) compared with $1.4 million ($2.11) last year.

Total deposits rose to $386 million on June 30 compared with $307 million a year earlier and directors voted to increase the quarterly dividend to 80 cents a share (from 65 cents), starting Aug. 1 to stockholders of record July 21.

Citizens Bank and Trust Co. of Maryland, based in Riverdale, also reported a sharp first-half gain in earnings. Net income rose 25 percent to $4.06 million ($3.17 a share) compared with $3.2 million ($2.54) last year.

The bank, which has 52 offices, reported June 30 deposits of $471.5 million compared with $445 million a year earlier while loan volume rose $26 million to $298 million.

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Hotel Investors, a Kensington real estate investment trust specializing in hotel properties, reported earnings in the nine months ended May 31 rose to $1.7 million ($1.11 a share) compared with $1.4 million (92 cents) in the same period a year earlier.

For the third quarter alone, net income was $573,605 (37 cents a share) compared with $495, 171 (32 cents). Directors voted an increased dividend of 45 cents a share, payable Aug. 1 to owners of record July 17.

Hotel Investors recently sold its interest in a Hilton Inn and acquired a $2 million second mortgage on a Holiday Inn in Orange, Calif., reducing non-earnings assests to $3.4 million from $6.7 million a year ago.

Union First National Bank of Washington and American Bank of Maryland, two area financial institutions controlled by Financial General Bankshares Inc., have reported significant gains in profitability during the first six months of 1978.

At Union First, earnings were $2.5 million ($1.09 a share) in the recent six months compared with $1.95 million (86 cents) in the same period last year. Second-quarter earnings rose to $1.3 million (58 cents) from $981,493 (43 cents).

Assets in the past 12 months rose 8 percent to a record $620 million on June 30.

American Bank, of Silver Spring, reported six-month profits of $783,000 ($1.04 a share) compared with $336,642, (44 cents) in the 1977 period. (In the second quarter alone, American earned $424,304 (56 cents) compared with $210,483 (28 cents).

President G. J. Manderfield said the higher earnings reflected a 45 percent increase in loan volume. Directors also approved a dividend of 15 cents a share, payable July 14 to stockholders of record June 30.

First National Bank of Southern Maryland, in Upper Marlboro, reported six-month earnings rose 6 percent to $483,502 ($1.49 a share) compared with $454,807 ($1.40) in the same period last year.

Central National Bank of Maryland, in Silver Spring, reported six-month earnings of $168,812 (90 cents a share), up 25 percent from $134,809 (72 cents) in the 1977 period. Not counting securities transactions, operating profits were up 40 percent to $153,971 (82 cents) vs. $110,074 (58 cents). Directors voted a cash dividend of 18 cents a share, paid July 10 to stockholders of record June 26.