Abstract The crustose tetrasporophyte of Acrosymphyton purpuriferum (J. Ag.) Sjost. (Rhodophyceae, Cryptonemiales) produces tetrasporangia in short day conditions (S.D.) and not in long day conditions (L.D.). At a temperature of 12°C in S.D. the first tetrasporangia are formed in 8 days; after 20 days all plant, have formed tetrasporangia. A tetrasporangium sporulates 4–5 days after its formation. A distinct response to S.D. is induced by as few as 6 S.D. cycles; 15 S.D. cycles are sufficient to induce a response in all plants. Plants in which a response has been induced by a number of S.D. cycles continue to produce tetrasporangia in L.D., but only in thallus parts which have been under S.D. conditions. The S.D. response is unaffected by light breaks of white light or narrow band coloured light. The number of fertile plants decreases gradually with increasing daylength over a rather broad range of light-dark regimes. The critical daylength (defined as the daylength at which 50% of the plants become ferti...