This note reports on incidental observations of the early development of the third-stage larvae of Hysterothylacium aduncum from gadid fishes. Gravid H. aduncum females were collected from Pollachius virens, Pollachius pollachius. Gadus morhua, and Molva molva in Norwegian waters. The eggs were incubated at 20 per thousand salinity and 5 C. Spontaneous hatching of third-stage larvae was observed 10-25 days after egg deposition. These larvae were long lived and could infect Acartia tonsa copepods, the infections being maintained for up to 34 days. The morphology of the third-stage larvae in the copepods and some traits of the life cycle were similar to those reported in previous studies. However, our results disagree with evidence suggesting that H. aduncum eggs rarely hatch, and hatched larvae have lower survival and a poorer ability to infect the first intermediate host than unhatched ones. It is difficult to account for these discrepancies because information on the early development of Hysterothylacium species is incomplete. However, we tentatively suggest that differences in the early development of H. aduncum may indicate the existence of at least 2 different taxonomic entities in the North Atlantic, which is consistent with previous evidence based on morphological traits.