The combined effects of water temperature, salinity, and pH on persistence of avian influenza virus (AIV) were evaluated in a model distilled-water system using three isolates from ducks sampled in Cameron Parish, Louisiana. Variables were tested within the ranges normally associated with surface water. Differences were detected between temperature (17 C and 28 C), pH (6.2, 7.2, 8.2), and salinity (0 ppt and 20 ppt), with a strong interactive effect observed between pH and salinity. Estimated persistence of infectivity for 1 x 10(6) mean tissue-culture infective dose of A/mottled duck/LA/38M/87 (H6N2) was longest at 17 C/0 ppt/pH 8.2 (100 days) and shortest at 28 C/20 ppt/pH 8.2 (9 days). Differences in the response to these variables were apparent between viruses. The ability of AIV to persist in surface water was also evaluated using samples collected from varied waterfowl habitats in coastal Louisiana. Observations were consistent with the model system, with duration of infectivity decreasing with increased salinity and pH. This suggests that experimental results may have application to field conditions.