The monogenean Benedenia seriolae (Yamaguti, 1934) Meserve, 1938 is a major pathogen of farmed yellowtail kingfish Seriola lalandi in South Australia. To control parasite populations in commercial farms, an understanding of the effect of water temperature on parasite development is vital. This study investigated the effect of water temperature (14, 18, 22 and 26 ± 0.5°C) on development, growth rates and age at sexual maturity of B. seriolae. Five distinct developmental stages, defined by the progressive development of reproductive organs, were used to describe development of B. seriolae from recently invaded larvae to sexually mature adults. Parasite age at the first sign of sexual maturity (ability to lay eggs) was strongly influenced by water temperature and was attained at 41, 24, 16 and 14 d post infection (p.i.) at 14, 18, 22 and 26 ± 0.5°C respectively. Four parameters (parasite total length, maximum parasite width, accessory sclerite length and anterior hamulus length) were examined for suitability as an index of parasite age. Growth rates for each parameter increased with water temperature; however, mean anterior hamulus length was identified as the most reliable index of parasite age. Equations derived from these data can be used to estimate parasite age and time to sexual maturity at water temperatures ranging from 14 to 26°C. The equations provide a simple tool to assist implementation of strategic treatment plans for B. seriolae infections in commercial kingfish farms, not only in South Australia but in other localities worldwide where S. lalandi is farmed. ; Julia A. Lackenby, Clinton B. Chambers, Ingo Ernst, Ian D. Whittington