1. Irradiation with three short ultraviolet (UV) wave lengths, 226, 233, and 239 mµ rapidly immobilizes Paramecium caudatum, the dosage required being smaller the shorter the wave length. 85 per cent of paramecia immobilized with wave length 226 mµ recover completely. Recovery from immobilizing doses is less the longer the wave length.
2. Irradiation continued after immobilization kills the paramecia in a manner which is markedly different for very short (226, 233, and 239 mµ) and longer (267 mµ) wave lengths.
3. An action spectrum for immobilization in P. caudatum was determined for the wave lengths 226, 233, 239, 248, and 267 mµ, and found to resemble the absorption of protein and lipide in the wave length region below 248 mµ. Addition of these data to those of Giese (1945 b) gives an action spectrum resembling the absorption by albumin-like protein.
4. Division of P. caudatum is delayed by doses of wave lengths 226, 233, and 239 mµ which cause immobilization, the longest wave length being most effective.
5. Immobilization at any of the wave lengths tested (226, 233, 239, 248, 267 mµ) is not photoreversible when UV-treated paramecia are concurrently illuminated.
6. Division delay resulting from immobilizing doses of 226, 233, and 239 mµ is photoreversible by exposure to visible light concurrently with the UV.
7. Division delay induced by exposure to wave length 267 mµ is reduced by exposure to visible light applied concurrently with UV or immediately afterwards.
8. The data suggest that the shortest UV wave length tested (226 mµ) affects the cytoplasm selectively, because it is absorbed superficially as indicated by unilateral fluorescence in UV. Consequently it immobilizes paramecia rapidly but has little effect on the division rate because little radiation reaches the nucleus.
9. The data support the view that nuclear effects of UV are readily photoreversed but cytoplasmic effects are not.