Irradiation of single axopodia near their distal ends with ultraviolet light induces axopodial retraction which continues for a short period after irradiation has ceased and appears to depend on the reduction in length of a microtubular axoneme. Altering the target site of the microbeam to the mid-point of an axopodium and prolonging the irradiation time results in the amputation of the distal portion of the axopodium. Both retraction and amputation may involve an ultraviolet light dependent breakdown of cytoplasmic microtubules. Axopodia grow out again after retraction and after amputation. The rates of growth of axopodia shortened in both these ways has been compared. Axopodia which have been caused to retract grow at a mean rate which is faster than that of axopodia which have been amputated.