Experiments show that the first of a series of replicate electric fishings can cause a decrease in catchability so that the second and subsequent catches are made from what is, effectively, a reduced population. Depletion (DeLury) estimates can be thus seriously low. Species affected are roach Rutilus rutilus (L.), gudgeon Gobio gobio (L.), rudd Scardinius erythophthalmus (L.) and, from other published data, probably rainbow trout Salmo gairdneri Richardson and eels Anguilla anguilla (L.). Being caught does not, apparently, cause fish to become less catchable. The reason why an electric field causes a proportion of a population to become uncatchable is obscure but it may be that refractory behaviour is involved; the effect lasts for between 3 and 24 h. A method is given for adjusting DeLury estimates for decreased catchability.