Variations in Irgarol 1051 concentrations in the UK’s largest marina at Brighton were determined regularly over a period of one year. Aqueous concentrations ranged from <1 to 960 ngl⁻¹ with highest mean concentrations generally associated with berths for larger vessels and with the main channels. Temporally, highest concentrations were recorded in November through to January and were probably associated with maintenance of vessels in an adjacent boatyard. Elevated levels were also encountered at the beginning of the season, coinciding with the introduction of newly antifouled vessels. Increased concentrations also followed dredging, possibly through re-mobilisation of Irgarol 1051. No correlations were found between dissolved Irgarol 1051 concentrations and pH, temperature or salinity. With the exception of sporadically high concentrations recorded for water samples (probably taken in close proximity to recently antifouled vessels), concentrations rarely exceeded the no observed effect concentration for marine periphyton of 63 ngl⁻¹. Concentrations of Irgarol 1051 in sediments sampled from the marina ranged from <1 to 77 ngg⁻¹. Apparent distribution coefficients (K_d) calculated from sedimentary and aqueous samples (collected simultaneously) are generally within the range of K_d’s reported from laboratory experiments.