The Marine Biological Association (1957) lists the poor cod Trisopterus minutus minutus (L.) (=Gadus minutus) as being parasitized by two monogenean gill parasites Diclidophora luscae and D. denticulata but at various times over the past 25 years we have, between us, examined the gills of upwards of 1000 specimens of T. m. minutus without ever having found a monogenean parasite. We concluded that the 1957 records were probably due to a misidentification of the hosts. However, in January 1979 one of us (J. E. G.) found several specimens of a monogenean very closely resembling D. luscae (Beneden & Hesse) on the gills of poor cod. D. luscae is normally found at Plymouth (Llewellyn, 1956) on only the pouting Trisopterus luscus (L.), a fish which from time to time is caught in the same trawl as T. m. minutus, e.g. in a total of 46 standard hauls between July 1976 and June 1978 (information supplied by Mr A. D. Mattacola), on 11 occasions (23%)T. luscus and T. m. minutus were caught together. On eight of these 11 occasions the lesser abundant of the two fish species constituted 10% or more of the combined total, and on three occasions the lesser abundant fish made up at least 40% of the combined total. Thus there is evidence that T. m. minutus and T. luscus from time to time occupy the same or neighbouring habitats (a standard haul may be over a distance of up to 8 km) and so presumably there has been opportunity for parasitic invasion from one species to the other to take place. That this has not in fact happened we have regarded as an example of the very strict species-specificity to their hosts manifested by most monogeneans and especially by diclidophorans on gadoids (Llewellyn & Tully, 1969). It was decided therefore to investigate how the sudden appearance in 1979 of a monogenean parasite on a previously uninfected population of poor cod may have come about.