Periphytic diatom samples from a metal-contaminated Rocky Mountain river in Colorado, U.S.A. were analyzed on two occasions for the presence of morphological abnormalites. Samples were collected from natural (rocks) and artificial (tiles) substrates at 12 sites displaying a range of metal concentrations. Members of the genus Fragilaria sensu Krammer and Lange-Bertalot (including Synedra and Hannaea), which was abundant at all of the sampling sites, exhibited the highest incidence of abnormalities. There were no significant differences in percentage of deformed cells between natural and artificial substrates. Percentage of diatom abnormalities at each site ranged from 0.2 ± 0.2% to 12.0 ± 2.0% of the total population, and normal Fragilaria valves were always observed along with abnormal valves for each taxa. Percentage of abnormal valves at a site was transformed (are sine square root) and regressed against dissolved cadmium, copper, iron, and zinc. For 1991, the best regression model fit (based on Mallows' Cp) was a two variable model using Cd and Zn (r20.39, Cp=2.4). In 1992, the four variables model (Cd, Cu, Fe, and Zn) provided the best fit to the Fragilaria data (r20.60, Cp=5.0). These data indicate that morphologically abnormal Fragilaria valves may be an indicator of elevated dissolved metal concentrations in streams.