The community-level effects of previous chemical exposure on resistance to a dissimilar subsequent stress were evaluated. Naturally derived periphytic communities on polyurethane-foam artificial substrates provided colonists for development of new communities under controlled stress conditions in the laboratory. Communities were colonized under three levels of Zn stress: control, low (73 pg/L), and high (172 Fg/L). After 21 d, laboratory-colonized communities were exposed to 5 pH levels (control, 4.5,4.0, 3.5, 3.0) for 48 h. No significant differences occurred between Zn treatments in response to pH for total biomass, assimilation ratio, respiration to biomass ratio, and net daily metabolism. Response to pH was dependent on Zn treatment for number of protozoan species, algal biomass, gross primary productivity, and community respiration. Number of protozoan species in high Zn was impaired in absence of pH stress. Primary production and community respiration were impaired in absence of pH stress in both low and high Zn. For these three end points, outcome for Zn treatments converged with increasing secondary stress; no differences between Zn treatment groups were observed at pH < 4.0. For several end points, Zn-stressed communities changed less, relative to their initial state, with subsequent acid shock.