In 1993 the potential of Scatella stagnalis Fall. to transmit the spores of Fusarium oxysporumf sp. lycopersici (Fol) in a protected soilless tomato crop was documented. The flies maggots feed mostly on blue-green algae and diatoms developping on the rockwool substrates. When feeding in the root zone of a Fol infected plant, larvae ingest pathogen microconidia which remain viable in the gut of pupae and adults. The latter transmit the inocula throughout the crop by means of the feaces. Occurrence of S. stagnalis in the substrates of imported tomato seedlings is common and may represent a potential risk in the transmission of various pathogens. Since the insect is practically unavoidable in established hydroponic systems, disinfecting the recirculating nutritional solution is of vital importance.