Bryum capillare, a moss of worldwide distribution, is ubiquitous to cities where it is commonly found growing within the cracks of sidewalks. Because of its occurrence in the harsh and depauperate urban environment, it was hypothesized that this moss is resistant to the deleterious effects of pollutants, such as acid rain. We tested this by measuring the growth response of an axenic culture line of B. capillare to simulated pH-adjusted rain. Field and experimental data both suggest that B. capillare is not especially resistant to acid rain. In the first experiment B. capillare protonematal cultures were subjected to simulated rain at pH levels between 3 and 8. Cultures at pH 4.0 simulated rain resulted in an approximate 50% reduction in colony area from that obtained in the pH 8.0 treatment. Therefore, we would expect the acid rain in Washington, DC to be unfavorable to the growth of B. capillare. Yet, the moss is both common and abundant in the city. In a second experiment the optimal pH for the in vitro culture of B. capillare ranged between 6 and 8. This helped explain its abundance in the city since soil pH samples averaged 7.5. We attributed the near neutral pH of soil under B. capillare colonies to its parent substratum: cement. In a third experiment, the adverse effects of simulated acid rain on B. capillare, were reversed by culturing the moss on cement-amended medium (pH 7.6). Based on these experiments and on the pH values of soil and rain samples, we concluded that B. capillare is protected from adverse effects of acid rain by the apparent buffering and neutralizing capacity of its preferred substrate, the cement of city sidewalks.