The acute effects of a herbicide based on 2,4‐D on embryos and larvae of bleak,Alburnus alburnusL. exposed to concentrations of 25–3200 mg 1−1for 12–48 hours were investigated. The 2,4‐D delayed or stopped the development of embryos in their early stages, caused behavioural changes and morphological alterations. The L.C.50values for embryos varied between 12–9 and 15·9–4mg 1−1and for larvae from 51·6 to 111·2mg 1−1according to the time of exposure. The level of 2,4‐D in Lake Balaton, or similar shallow lakes, without substantial injury of the littoral fish fauna should not