Groups of rotifers of the species Asplanchna brightwelli were exposed to U.V. irradiation at dosages ranging from 50 to 4800 J/m2. The lifespan of the rotifers was significantly reduced by exposure to U.V. in the range of 200 to 4800 J/m2. A logarithmic decline in lifespan was seen as the U.V. dose increased. Rotifers were most sensitive to U.V. exposure during the prereproductive stage at the beginning of their lifecycle. Exposure of rotifers to visible light following U.V. radiation provided no evidence that photoreactivation could influence lifespan in this species.