Abstract: Soil-dwelling predatory mites are important predators of sciarid flies (Bradysia spp.).The predatory mite Macrocheles robustulus, has been commercially available since 2010. Theeffectiveness of this predator in the control of sciarid flies has, however, not yet been tested. Inthis study we compare the effectiveness of M. robustulus and the frequently used Hypoaspisaculeifer in controlling sciarids in potted chrysanthemum under greenhouse conditions. We alsoevaluate the potential of using a mulch layer to improve establishment, population increase andperformance of the predators. Both predators had a significant impact on sciarid densities, with areduction of 97.1% by M. robustulus and 87.1% by H. aculeifer. When the predators wereintroduced in combination with a mulch layer of Biotop®, predator densities increased by a factor3.1 for M. robustulus and 11 for H. aculeifer. The increase of predatory mite density wasassociated with an increase in the density of astigmatid mites, on which the predators were rearedand that were introduced simultaneously with the predators. Sciarid density was reduced by99.5%, when M. robustulus was introduced together with the Biotop® mulch layer, significantlylower compared to treatments with H. aculeifer with or without Biotop®. These resultsdemonstrate that M. robustulus is an effective predator of Bradysia spp. and that in combinationwith Biotop® it provides better control than the frequently used H. aculeifer.