AbstractDichelesthium oblongum belongs to the family Dichelesthiidae, within the Suborder Siphonostomatoida, subclass Copepoda (Kabata 1979; Huys and Boxshall 1991). A description of the life cycle and the morphology of its developmental stages is described and compared with that of other organisms within this family and suborder. As expected within the Dichelesthiidae, Dichelesthium oblongum, life cycle appears to consist of nine stages, from nauplius 1 to adult. Sexes are easily distinguished from the 1st copepodid stage on the host through adulthood. General trends include the development of the lateral process on the endopod of the second leg on the male, the drastic elongation of the genital complex of the female, and an increase in size of all appendages in both the male and females through maturation. Atlantic sturgeon are an ancient fish which are parasitized by the marine ectoparasite Dichelesthium oblongum. These parasites have been observed on sturgeon in marine habitats in Europe and on the east coast of North America. D. oblongum were sampled from Atlantic sturgeon along the Atlantic coasts of New York, Connecticut and Delaware from 2007-2010. The life stages of individual parasites were determined. A high infection pressure for this parasite in the late summer/early autumn was observed for Atlantic sturgeon, and based on abundance, prevalence and staging data the Jones Beach sampling area appeared to have the highest infection pressure, potentially even acting as a major source of infection. Prevalence and abundance of D. oblongum was higher along the South shore of New York than Delaware, despite larger fish caught in the latter location, yet this may be a function of temporal differences between the sampling. Using this information, a generation time for D.oblongum was estimated. This information will be useful in determining a epidemiological model for this host-parasite relationship.