Early life history stages of brook trout were exposed abruptly to low pH in laboratory tests to simulate pH depressions in streams during the spring runoff; growth, development, and mortality were monitored. Eyed eggs showed delayed hatching and increased mortality during hatching at pH 5.5, compared with control eggs at pH 6.9. Posthatch growth patterns did not differ at pH 5.5 and above. At pH 5.0, however, growth of larvae was retarded in coincidence with delayed development of the gastrointestinal tract. Both growth and gut development rebounded during the juvenile stage, an indication of acclimation of the growth response to low pH. At pH 4.5, growth was curtailed and alevins suffered 100% mortality when gill filament expansion occurred at higher pHs. Biomass of treatments during the 123-d study differed as follows: Control = pH 6.5 = 6.0 > 5.5 > 5.0 > 4.5. When acidification was imposed at the alevin stage, a reduction to pH 5.0 caused immediate growth retardation, followed by a period of growth comparable to that of the control. At pH 4.5, alevins exhibited growth well below control levels throughout this experiment (26 d). Acidification at swim-up to pH 4.5 did not affect feeding behavior of larvae, but growth was retarded. Later life stages were more resistant to exposure to low pH; eyed eggs, alevins, and larvae exposed to pH 4.5 exhibited 100, 48, and 13% mortality, respectively. Acidic spring runoff is a threat to brook trout productivity in streams.