Unionids, which have parasitic glochidia larvae, likely compete for host fishes in multispecies mussel beds (> 20 species). The timing of glochidia abundance and the potential for partitioning of their ‘host-fish’ environment were examined. Glochidia were continuously sampled at 2-h intervals over ten 24-h days using a rosette sampler in the Sydenham River, Ontario from late August to September 2020. A total of 6104 glochidia from 17 unionid species were analyzed. A generalized linear mixed model indicated that the highest abundance of glochidia was at night, peaking at 20:00 (± 1 h) LST. Chi-square analysis revealed that many species were not uniformly distributed throughout the day, rather, their abundance aligned with reported diel activity patterns of their hosts. Glochidia abundance peaked at different times in unionids that shared hosts, suggesting host partitioning. Results provide insight into the concept of host partitioning to reduce competition, which will aid in unionid conservation.