Arlington Sheriff James A. Gondles Jr. announced yesterday that his office had been electronically bugged.

At a news conference on the front steps of the Arlington County Court House, Gondles said a listening device was found last week by a private security specialist and that the Arlington Police Department is conducting an investigation at his request.

Gondles said his suspicions that his office might be bugged were aroused several weeks ago. He would not elaborate on why.

Gondles said he is "outraged by the invasion of my privacy," though he said he is unsure whether the device had been activated. A check by the telephone company shows his home telephone also might have been tampered with, he said.

Arlington police spokesman Tom Bell said the bugging, which is a felony, is under investigation. Bell said detectives are trying to trace the sale of the device.

Gondles said he did not know who was responsible for planting the bug. However, in a written statement he described how the microphone was attached to a wire leading to a phone plate "beneath a desk once occupied by my former chief deputy and opponent in this year's election."

Gondles, a Democrat, is being challenged by Republican-backed independent Ronald B. Hager. Hager resigned as Gondles' second-in-command in March to run for sheriff, accusing his former boss of mismanagement and excessive travel.

Hager denied any involvement in the eavesdropping incident. "I was shocked to hear of the sheriff's allegations of a listening device being found in the wall between his office and mine," Hager said.

Hager added that he welcomed a police investigation. Hager then said he has been the victim of frequent crank calls at 3 a.m. "People don't just hang the phone, they slam it down," he said. "I've asked for a tracing device on my line."

Gondles' statements yesterday added another twist to what has become an increasingly curious and contentious campaign for sheriff. "Prior to March 6," the day Hager resigned, "you could say we were friends," said Gondles.

In recent weeks charges and countercharges have been exchanged between the two and their representatives.

Hager has accused Gondles of conflict of interest in the purchase of $3,000 of athletic clothing for the county jail from Gondles' father-in-law. Gondles has said the purchases were initiated by Hager when he was chief deputy.

This week, Arlington Treasurer Francis X. O'Leary, a Democrat, accused Hager of violating state and county laws by using a middleman to purchase a truck sold for $25 at a sheriff's auction. Hager has denied any wrongdoing.

Gondles, who hired the security firm that searched his office, said no county funds were used firm for the inspection.