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The effects of host size and host species on the prevalence and fitness of conopid flies (Diptera: Conopidae) parasitizing bumblebees (Hymenoptera: Apidae) were investigated. Field data from nine sites across south-western Alberta, Canada, showed that conopids parasitized, on average, 12.3% of workers and 3.5% of male bees. In general, bee mass was a better predictor of host use and conopid offspring size than bee species. Host mass could not, however, explain the very low prevalence of conopids in the long-tongued bumblebee, Bombus californicus Smith, or in male bees in general. Conopids predominately infested bees of intermediate size, and as a result, occurred most commonly in the intermediately sized species, B. flavifrons Cresson. Host quality, in terms of conopid offspring size, increased as a non-linear function of bee size. The results are discussed with respect to the impact of conopids on bumblebee populations, and the relation between host quality and patterns of host use. [ABSTRACT FROM AUTHOR]