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The results of experiments with white clover (Trifolium repens) in which time of nodulation and seedling plant weight or vigour were measured are reported. Experiments 1 and 2 were conducted in artificial growth medium in test tubes with controlled inoculation and experiment 3 in soil without controlled inoculation. Experiment 1 which was preliminary in nature showed the extent of the variation for time of nodulation after inoculation with Rhizobium trifolii. It was evident also that plant vigour and the number of days to nodulation were negatively correlated. Experiment 2 forms the major part of the results and is concerned with the analyses and interpretation of the diallel cross progeny of twelve plants selected from experiment 1. The results indicated a rather complex genetic picture for the two characters measured, namely days to nodulation and seedling plant weight at 80 days. Reciprocal (both general and specific) as well as additive (g.c.a.) and non-additive (s.c.a.) effects were present. Experiment 3, in which seed from 22 families of the diallel cross was sown in soil without controlled inoculation, indicated that the results obtained under the laboratory conditions of experiment 2 and those obtained in soil conditions were not correlated. The implications of these results in relation to selection of improved varieties of the host species are discussed.