Attack on oats by Heterodera avenae, on white clover by H. trifolii, and on perennial rye grass and on oats by Tylenchorhynchus dubius did not affect water consumption/g plant/day or dry matter content of plants directly at population densities smaller than 25 T (T as in y = m + (1-m) ZP-T (1) for P T, where y = relative plant weight, P = nematode density, z-T = 1. 05 and m = a constant T and the other at densities > 25 T. Two mechanisms of growth reduction could also be postulated for oats infected with H. avenae, for perennial rye grass with T. dubius and for white clover with H. trifolii. Three different mechanisms by which nematode attack interferes with the growth of plants are distinguished, causing: (1) reduction of the rate of growth and development, without further changes, (2) growth reduction associated with a reduction of water consumption/g plant/day and increase of dry matter content and (3) increase of the shoot weight to root weight ratio and decrease of water consumption/g plant/day with reduction of the rate of growth as a possible consequence.