A third-order reach of Norris Brook, a small stream in the Hubbard Brook Experimental Forest, was experimentally acidified for five months to determine what effects acid precipitation may have on the ecology of a poorly buffered lotic ecosystem. The initial six days of stream acidification simulated a low pH regime that can occur in a small mountain stream receiving meltwater from a rapidly thawing snowpack contaminated with acidic deposition. The first two to three days of acid addition constituted a period of acute H-ion stress (pH 4) that elicited a tenfold increase in the daily drift rate of benthic macroinvertebrates. This increased drift leaving the acidified reach was also more diverse overall in terms of major taxa (orders), trophic functional groups, and behavioral groups but less diverse at the generic level than the drift entering. In addition, the macrofauna abandoning the acidified area compared to that entering was particularly more diverse generically in mayflies and probably midges, collector-gatherers, and clingers and swimmers.