Egg-laying patterns and egg production in Heteraxine heterocerca from the gills and Benedenia seriolae from the skin of Japanese yellowtail Seriola quinqueradiata in Japan were investigated in vivo. Eggs were collected every 3 h from sexually mature H. heterocerca and B. seriolae infecting 3 S. quinqueradiata kept individually over 3 consecutive days and exposed to alternating periods of illumination and darkness (LD 12:12; light on 06.00, light off 18.00) and maintained at 23·8±0·1°C and 35 ppt salinity. A well-defined egg-laying rhythm was demonstrated for H. heterocerca while B. seriolae was shown to release eggs continuously. A total of 114 000 H. heterocerca eggs was collected and of these, 45·4 (42·5–49·9)% were collected during the first 3 h period following dark at 18.00 h. A total of 662 857 B. seriolae eggs was collected and these were distributed over each 3 h period ranging from 11·1 to 14·1% of the daily egg output. All eggs extracted from the uterus of each H. heterocerca were joined together forming an ‘egg-string’. The contrasting egg-laying patterns of H. heterocerca and B. seriolae suggest that each species makes use of a different infection strategy to infect the same host species, S. quinqueradiata.