Abstract: Low humidity levels are assumed to be an important limiting factor for biologicalcontrol of spider mites by phytoseiid predatory mites, mainly because of the vulnerability ofthe egg stage for drought. In this study, we evaluated the efficacy of several species ofphythoseiid predatory mites for control of the spider mite Tetranychus urticae Koch at low(55%) and high (80%) humidity levels on greenhouse cucumber in two greenhouse trials wherepredators were released either before or after spider mite introductions. In the preventive trial,the best control was achieved by Neoseiulus californicus McGregor at both humidity levels.Spider mites were completely eradicated. None of the other species of predatory mites was ableto reduce spider mite densities, except Transeius montdorensis (Schicha) at the low humiditylevel. In the curative trial, N. californicus was again the best performing predatory mite,together with Phytoseiulus persimilis Athias-Henriot. Spider mites were completely controlledat both humidity levels by these species. This suggests that low humidity levels during daytimedo not necessarily disrupt the control of spider mites by these predators, possibly because ofthe humidity fluctuations between day and night. Humidity did also directly affect spider mites;densities were up to 3 times higher on plants in the greenhouse with low humidity levels thanon plants with a high humidity level. This increased growth rate of spider mites at low humiditylevels may be another explanation for the failure of spider mite control in practice. Yet, in orderto enhance the biological control of spider mites at low humidity levels, it might be better tofocus on methods that support the establishment of effective spider mite predators, rather thanselecting drought adapted strains of predatory mites.