A public offering of 400,000 shares of common stock has been filed with the Securities and Exchange Commission by Essex Corp., an Alexandria-based behavioral sciences and engineering services research firm.

According to Johnston-Lemon & Co. Inc., which will underwrite the offer, the issue will be made available in mid- to late September. In addition, another 130,000 shares will be offered by certain selling stockholders.

Founded in 1969, Essex Corp. performs work for clients such as the federal government, state governments, the Navy, Army and Marine Corps, public utilities, private domestic and foreign companies. Sixty to 70 percent of its contracts are government-related.

The firm is a major contractor on the Trident submarine program, providing logistics support and planning services. Another major segment of its business is "human factors engineering" for the nation's nuclear power plants.

After the accident at Three Mile Island, Essex was called in to study design flaws in the reactor control room and redesign it to reduce the chances of human error.

Essex employs 230 people and has several offices nationwide. Company officials have projected substantial increases in revenues and earnings for the current year based on increases in defense contracting and private work.