Abstract: healthy host clones and 14 clones originally diseased with the annual rust, Uromyces trifolii-repentis, were sampled from the natural population of Trifolium repens, which were used as the experimental plant materials. 10 rust strains of Uromyces trifolii-repentis were collected for the experimental inoculation. Two greenhouse and two field experiments were done, with three treatments as inoculated with one rust strain, inoculated with ten rust strains and controls, respectively. The results showed that originally healthy plants were more resistant, and originally diseased ones were more susceptible to the rust. However, there was a considerable overlap among some of the clones. Pot experiments, both in the greenhouse and in the field, revealed no significant differences in growth between originally healthy and originally diseased plants within the treatments (controls, inoculations with one strain and with ten strains). However, there were significant differences in growth between treatments. This lack of difference in growth within treatments contrasted to the result of a transplantation experiment in a natural meadow where the rust was absent. The transplantation experiment showed that originally diseased clones produced significantly more leaves than originally healthy ones (mean +S.E.:+0.77 and 17.6+0.49, respectively). Further, originally diseased clones showed more variation in leaf production, compared with the originally healthy ones. The difference between the pot experiments and the transplatation experiments tells that to study individual differentiation in responses to environmental factors, such experiments ought to be performed under a range of natural conditions, where various biotic and abiotic factors are involved.