Changes in the microbial flora associated with the skin and gill tissue were used to detect pollution- and parasite-related stress in Girella nigricans (Opaleye perch), a rocky reef fish. Enumeration studies revealed significantly greater densities of adherent bacteria on gill filaments of fish maintained under polluted water conditions in comparison to fish maintained under "clean" water conditions, over a 6 1/2 week period.  When an additional stress factor was applied in the form of a parasitic infection (Parafilaroides decorus), a further significant increase in adherent bacteria densities was observed on the gill filaments.  A significant increase in bacterial densities on the skin was only noted when both stress factors were applied simultaneously.  Bacteria of the genus Pseudomonas were the dominant isolates recovered from both the gill filaments and skin of non-parasitized fish at the time of capture.  After exposure to polluted water for 6 1/2 weeks following capture, the number of pseudomonads in relation to other adherent bacterial groups further increased. Upon inoculation and infection of the fish with P. decorus, no substantial shift in-the bacterial population of the skin or gill filaments was observed. Thus increased bacterial densities and shifts in the population structure may reflect a subtle impairment of the host defense mechanisms) resulting from pollution- and parasite-related stress.