Deutonymphs of Digamasellus fallax Leitner, phoretic on the females of the sciarid Lycoriella aitripila Winn., fed and developed on saprophagous nematodes but not on eggs or larvae of the host. Extraction of compost in which sciarids develop showed a high initial population of nematodes. The introduction of phoretic nymphs at oviposition of the host would therefore synchronise with the rapid increase in their prey. Comparison is made with phoresy in Arctoseius cetratits (Sellnick) which is a predator of the sciarid host.