Abstract Gel- and topcoat surface layers on fiberglass [glass-reinforced plastic (GRP)] made of unsaturated resin based on isophthalic acid polyester and neopentyl glycol (ISO-NPG) were tested for leaching, ecotoxicity of water eluates, and abrasion by river sediments at a current speed of 0.5 m * s −1. Leaching from topcoat tempered at low temperature was significant, whereas it was negligible from highly tempered gelcoat. Water eluates from both gel-and topcoat were nontoxic in routinely employed biotests (bacteria, algae, daphnids). No abrasion by river sediments was detectable. Based on these results, GRP with gelcoat made of ISO-NPG is considered a suitable building material for mesocosms.