Dorothy Yerkes opened her most recent bill from The Washington Suburban Sanitary Commission, the agency that supplies water to her Montgomery County home, and became flushed with anger.

"I never expected to find a bill for $900," she said tartly.

Actually, the bill was for $929.79, based on an average consumption rate of 3,000 gallons a day, a rate that implied someone had spent the last three months continually flushing the toilet while the shower ran in Yerkes' home.

The $929 bill was not such a surprise, however, since the bill before that had been $476, also based on the 3,000-gallon-a-day rate.

"At first I thought it was a joke, a computer error. I called them up and they said they'd done it deliberately so they could get my attention," she said.

They got her attention. Yerkes fired off a two-page, typed letter to Montgomery County Executive James P. Gleason criticising the WSSC as a "politically controlled monopoly" that was "blackmailing" her. Gleason made the letter available to the press, and the WSSC had to explain its novel customer-relations policy.

"We've been trying to get inside her house for the last 10 months in order to make a direct reading from her (basement) meter," said WSSC spokesman Arthur P. Brigham.

Direct readings are needed for the WSSC's new billing system, and are far more precise than the estimated usage system the WSSC had been using for Yerkes since its last direct meter reading at her home in March 1967, Brigham said.

"We've sent her two letters, asking her to contact us, but she never has. There's no formula, it's just an exaggerated bill," Brigham added.

That is not exactly the way Yerkes, a single used of water, sees it. "I've never received any letters from them. This is just blackmail and intimidation. The proof of it is that they've used this system on other people and were successful with it," she said.

In the past nine months the WSSC has sent out attention-getter bills to the 200 of its 240,000 customers who have failed to respond to requests for direct meter readings, Brigham said. He added that payment was never expected for the $929 bill.

"She sent back those cards which you mark yourself to show the meter settings, but if we had billed her on the basis of the markings the bill would have been far higher than $900," he said.

Yerkes said she will contact the WSSC on Monday, but can't figure out why a meter reader would want to come into her house anyway.