Avian botulism is an important disease affecting many species of waterbirds in North America, but the environmental conditions that initiate outbreaks are poorly understood. To determine wetland attributes associated with outbreaks of avian botulism in waterbirds at the Sacramento National Wildlife Refuge (SNWR), California, we compared environmental characteristics between wetlands where outbreaks occurred (outbreak wetlands) and did not occur (nonoutbreak wetlands). In June through October 1987-89, we monitored the occurrence of avian botulism via observations for sick or dead sentinel mallards (Anas platyrhynchos) placed in 4 wetland enclosures. During this same time period, we collected environmental data from the water column and sediments of each wetland enclosure at 10-14-day sampling intervals. Multivariate analysis was used to reduce 22 environmental variables to 7 factors for inclusion in subsequent statistical analyses. We found that outbreak wetlands had significantly lower redox potential than nonoutbreak wetlands. The probability of botulism in sentinel mallards was associated with increasing temperature, increasing invertebrate abundance or biomass, and decreasing turbidity. However, because these factors were not consistently higher in outbreak wetlands compared to nonoutbreak wetlands, they may have a more proximate effect in initiating an outbreak.