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Despite the importance of aquatic symbionts in community organization, often little is known about the relative role of host and environmental characteristics on symbiont abundance. We investigated the importance of environmental and host physical characteristics on the abundance of the branchiobdellidan, Branchiobdella kozarovi , on a population of narrow-clawed crayfish, Astacus leptodactylus , in the Islamic Republic of Iran. Crayfish from four sites in the Aras Dam Reservoir were sampled on 12 occasions during 2009. Abundance of B. kozarovi was modeled against water temperature, crayfish length, mass, and sex after controlling for temporal and spatial effects. The most parsimonious model indicates that B. kozarovi abundance was primarily related to water temperature but also increased linearly with host body length. We found that omitting water temperature dramatically decreased model performance while omitting host physical characteristics decreased model performance marginally. We recommend that ecologists consider both environmental and host physical characteristics when monitoring aquatic symbionts such as branchiobdellidans, as both types of factors will provide insight into community dynamics and guide future management strategies.