Cellular phone subscriptions grew nearly 25 percent in the first half of 1990 despite economic hard times in some industries that depend on mobile communications, an industry group said yesterday.

"In the face of a quivering economy, cellular continues to be extremely strong," said Robert W. Maher, president of the Cellular Telecommunications Industry Association.

The survey of 552 cellular companies found that 859,742 subscribers were added from Jan. 1 to June 30, an increase of 24.5 percent. By comparison, 817,151 subscribers were added from June 1989 through December 1989, a growth rate of 30.3 percent.

However, the average bill paid by subscribers of the 4.4 million cellular phones declined slightly in the first six months of the year, to $83.94 from $89.30, while the average length of calls remained about 2.3 minutes, according to an association survey.

A cellular phone increasingly is considered an essential item, not a luxury, by industries such as construction and real estate, which both have been hit hard recently with business slowdowns, Maher said.

"It shows increasing public and business acceptance of cellular at a time when the economy was not growing very much," said Charles J. Many, president of NYNEX Mobile Communications and chairman of the association.

Maher said the industry no longer was experiencing the huge growth rates of the mid-1980s when cellular first began, but that its growth was steady. Any declines have been more in the use of the phones than in subscribers, he said.

"Companies facing a little problem with the bottom line are probably cautioning on the use of cellular, to make sure it's business only," Maher said.

Industry service revenue was up 10 percent in the first six months of this year, to $2.13 billion.

The industry has grown from 32 systems in December 1984, representing a total capital investment of $354.8 million, to 592 systems with a total capital investment of $5.2 billion at the end of June. The $5.2 billion is 16 percent above December 1989.