Autotropism was examined in germinating spore pairs of Rhizopus stolonifer, Mucor plumbeus, Trichoderma viride, and Botrytis cinerea. When germinated on agar surfaces the first three species exhibited negative autotropism, B. cinerea being neutral in its autotropic behaviour. More pronounced negative autotropism was shown by the first three species when germinated on a film of Cellophane applied to an agar surface. Under these conditions B. cinerea displayed positive autotropism. Spore pairs of R. stolonifer germinated on agar containing cellulose powder or charcoal showed less negative autotropism than on agar alone. Touching spore pairs of each species showed a marked tendency towards cis-ness, i.e. germ-tubes beginning on the same side of a line joining the two spore centres, under the culture conditions described, the one exception being the reduction in cis-ness recorded when R. stolonifer was germinated on agar containing charcoal. Time courses of germination were determined for single spores and touching spore pairs of R. stolonifer and M. plumbeus and a significant promotion was obtained in the spore pairs as compared with the single spores. Although both these species exhibit marked negative autotropism there was a strong tendency for the positive germ-tube, i.e. one beginning more nearly towards its neighbour, to emerge before the negative germ-tube in those spore pairs having one germtube positive and the other negative in orientation. Also, in R. stolonifer, the replacement of germination-promoters by germination-inhibitors in filtrates from spore suspensions as they age is correlated with a change from positive to negative autotropism in germinating members of (+ −) spore pairs. Possible mechanisms are discussed to account for the observed effects.