The mating behaviour and reproductive mechanisms for two phytoseiid predators, Phytoseiulus persimilis Athias-Henriot and Amblyseius andersoni (Chant) were observed in the laboratory in Canada. The different mating patterns of the two species appeared to reflect their generic differences. In both species, female fecundity increased with the duration of copulation. Most males of P. persimilis inseminated females with a single endospermatophore transferred to only one of the female spermathecae, whereas males of A. andersoni transferred two endospermatophores, one to each spermatheca, in single matings that lasted for the full duration.