Irgarol 1051 (2-(tert-butylamino)-4-cyclopropylamino)-6-(methylthio)-1,3,5-triazine) is a triazine herbicide that is increasingly being used to boost the effectiveness of antifouling paints. Estuarine plants, such as the marine angiosperm Zostera marina L. (eelgrass) may accumulate, and be affected by, Irgarol 1051, in locations with high boat densities. Bioconcentration of Irgarol 1051 within Zostera tissue was determined in field plants and laboratory semi-static exposure experiments. Effects of Irgarol 1051 upon the growth rate and photosystem II photosynthetic efficiency of Zostera were examined over a concentration range of 0 to 25 mg dm − 3 . Growth rate was assessed by comparison of leaf specific biomass ratios, and was found to be reduced at and above an Irgarol 1051 concentration of 10 mg dm − 3 . Photosynthetic efficiency was assessed using fluorescence induction kinetics: efficiency was signifi-cantly reduced at 0.18 mg dm − 3 (0.4 mg g − 1 dry weight leaf tissue) and a 10-day EC 50 value of 2.5 mg dm − 3 (1.1 mg g − 1) calculated. Longer-term exposure revealed a 36-day EC 50 value of 0.2 mg dm − 3 . Uptake of Irgarol 1051 was rapid within the Zostera leaves: tissue concentrations (dry weight basis) in excess of 300 times the water concentration were found within 2 days of exposure. Leaf concentrations in excess of 14 times root tissue concentration were found. Estuaries sampled in S.W. England had low aqueous Irgarol 1051 contamination, typically B 0.003 mg dm − 3 , but Zostera leaf tissue concentrations (dry weight basis) were up to 25000 times the aqueous values; this was only 15 times below the 10-day EC 50 value. The reported results will enable the level of risk to isolated Zostera meadows from Irgarol 1051 to be assessed based on leaf tissue concentration and also have implications for the siting of marinas. © 1999 Elsevier Science B.V. All rights reserved.