An indirect fluorescent antibody technique (IFAT) has been developed to detect Hemato­ dinium sp. in the haemolymph and tissues of the Norway lobster Nepiiiops norvegicus. The IFAT, being more sensitive and reliable than previously used field and laboratory diagnostic techniques, detects both lower-level haemolymph infections as well as previously undiagnosable tissue infections. Low- level haemolymph and organ Hematodinium sp. infections have been found in apparently uninfected lobsters at all times of year, including late summer and autumn, when the parasite was previously thought to be absent from host lobsters. Currently, IFAT is routinely used for laboratory studies and the calibration of field diagnostic techniques.