The causes of sex determination in amphipods are believed to be multi-factorial. Sex determination in Echinogammarus marinus (Leach, 1815) has been reported to be linked with feminising parasites. To date, however, no such studies have linked this species with environmental sex determination (ESD). A field study and laboratory breeding experiments were conducted to determine the influence of photoperiod on sex determination. Over the two-year field study, males dominated during August to November, whilst female-biased populations were observed during April to July. A significant linear relationship was observed between photoperiods and sex ratios from the field data. Under laboratory conditions, photoperiod was also shown to be an influential factor in sex determination, with a male bias over a long-day photo regime (61.5 % male broods) and a female biased over a short-day photoperiod regime (43.5 % male broods). Findings suggest that there is some level of ESD present within the species, suggesting considerable plasticity in the sex differentiation pathway.