Single chromosomal spindle fibres in Nephrotoma suturalis (crane-fly) spermatocytes in metaphase and anaphase were irradiated with monochromatic ultraviolet light focussed to a 2 micrometer spot. In cells in both metaphase and anaphase either the birefringence of the irradiated spindle fibre was altered in the irradiated region, or there was no change, depending on the dose and wavelength of ultraviolet light used for the irradiation. When there was an area of reduced birefringence (ARB), it moved poleward regardless of whether the associated chromosome moved poleward. When cells were irradiated in early metaphase they remained in metaphase until the ARB reached the pole. In some cells irradiated in late metaphase the chromosomes began anaphase before the ARB reached the pole; in many such cells anaphase was abnormal in that all six half-bivalents separated at the start of anaphase but none moved polewards. In all cases the ARB moved poleward at the same speed as subsequent chromosome movement; that is, at about 0.8 micrometer/min. In cells irradiated in anaphase, spindle fibre birefringence was reduced independently of blockage of chromosome movement. Because birefringence and movement were altered independently there were four classes of results: (1) in some cases there was no effect on the movement of the chromosome associated with the irradiated spindle fibre and no effect on the birefringence of the irradiated spindle fibre. (2)In some cases, primarily with 260 nm wavelength light, there was no effect on the movement of the chromosome associated with the irradiated spindle fibre and there was an effect on the birefringence of the irradiated spindle fibre. (3) In some cases, primarily with 290 nm wavelength light, there was an effect on the movement of the chromosome associated with the irradiated spindle fibre and no effect on the birefringence of the irradiated spindle fibre. (4) In some cases, primarily with 270 nm and 280 nm wavelength light, there was an effect on the movement of the chromosomes associated with the irradiated spindle fibre and there was an effect on the birefringence of the irradiated spindle fibre. The action spectrum for reducing spindle fibre birefringence in crane-fly spermatocytes had two peaks, one at 260 nm and the other, less sensitive, at 280 nm. For irradiations at 270 nm, 280 nm and 290 nm, five to fifty times more energy was needed to reduce spindle fibre birefringence than to stop chromosome movement, but for irradiations at 260 nm five times less energy was needed to reduce spindle fibre birefringence than to stop chromosome movement. The action spectrum for reducing spindle fibre birefringence is quite different from that for stopping chromosome movement.