Ectomycorrhizae developed on four control-pollinated, half-sib progenies of slash pine (Pinus elliottii Engelm. var. elliottii) inoculated with Pisolithus tinctorius and on six progenies plus a mixed seed lot inoculated with Thelephora terrestris in aseptic culture. Host genotype influenced the amount of ectomycorrhizal development as well as the growth response of the host plants to mycorrhizae. Among progenies inoculated with P. tinctorius, 78 to 82 percent of the feeder roots of three progenies were mycorrhizal. Significantly fewer feeder roots (53 percent) were mycorrhizal in the fourth progeny, although the total number of feeder roots did not vary significantly among the four progenies. There were no significant differences in relative numbers of mycorrhizae (42 to 50 percent) among the six test progenies and the mixed seed lot inoculated with T. terrestris. Among the nonmycorrhizal controls, certain progenies developed significantly fewer feeder roots and less foliar weight than did others. However, when the progenies were mycorrhizal, these differences in growth response were minimal. Forest Sci. 17: 488-492.