Eggs of Neoheterocotyle rhinobatidis, Troglocephalus rhinobatidis and Merizocotyle icopae (Monogenea: Monocotylidae) from the shovelnose ray Rhinobatos typus (Rhinobatidae) have a distinct hatching pattern linked to light periodicity. Larvae of these 3 monogenean species emerge only during daylight when exposed to natural illumination or when incubated in alternating 12 h periods of light and dark (light on 06.00 h, light off 18.00 h). N. rhinobatidis larvae emerge with a distinct peak during the first 2 h of light; this peak is not as pronounced in T. rhinobatidis or M. icopae. Eggs of N. rhinobatidis incubated in a reverse light/dark cycle (light on 18.00 h, light off 06.00 h) hatched only during periods of illumination, again with a peak during the first 2 h of light. Evidence suggests that the hatching patterns observed in all 3 species represent true circadian rhythms because eggs incubated in 24 h light or 24 h dark conditions continued to hatch with a rhythm. Shadows, disturbance and host tissue did not promote hatching in N. rhinobatidis or T. rhinobatidis but there were indications that host tissue may promote hatching in M. icopae. The hatching patterns observed are discussed with respect to their adaptive responses to host behaviour and predation pressure.