(FORTUNE Magazine) – Summertime securities trading is typically slow, but the doldrums have becalmed Wall Street even more than usual this year. In July and early August * the average weekly volume on the New York Stock Exchange was down about 15% from the comparable period last year, and it would have been down a lot more if not for brisk ''dividend capture'' trading by Japanese institutions, which buy and sell stocks around the time they pay dividends, a procedure that earns favorable tax treatment in Japan. The Securities Industry Association figures that such trades, up sharply this year, accounted for 15% of volume.

What's going on? With stock prices down an average of 20% in the past six months, many investors look at recently increased interest rates and renewed inflation fears and understandably hesitate. Besides, who wants to jump into such a thin market, where one big trade can send prices who knows where? Michael Metz, a market analyst with Oppenheimer & Co., believes that ''what's happening on Wall Street is a self-fulfilling prophecy. The low volume tends to scare off potential participants,'' further lowering volume, scaring more investors, and so on. In at least one corner of the markets, trading has been hot in every sense. Investors gambling on the drought's longevity have sent commodity prices wildly up and down. On the Chicago Board of Trade, volume in July rose 23% to 11.6 million contracts, setting a new record.

CHART: NOT AVAILABLE CREDIT: ANDERS WENGREN CAPTION: NYSE TRADING VOLUME DESCRIPTION: Comparison of New York Stock Exchange daily average trading volume from 1st week in July through Wednesday of 2nd week in August for 1987, 1988.