Environmental and hormonal control of flowering and vivipary in four Norwegian timothy (Phleum pratenseL.) cultivars has been studied in phytotron and by aseptic culture of inflorescence explants. The critical photoperiod for flowering increased with increasing temperature (12–18°C) and it was 13 to 15 h for the southern and 14 to 16.5 h for the northern cultivars. Diurnal temperature fluctuation significantly stimulated flower formation compared to the corresponding constant temperature treatment. Plants grown in 16‐h photoperiod contained normal sexual flowers, but a high percentage of spikes developed in 12‐ or 14‐h photoperiod contained viviparous plantlets. One‐ to four‐weeks in continuous light before treatment with 12‐h photoperiod increased the number of spikes per plant, but did not enhance the frequency of vivipary. Experiments with aseptic cultures showed that generative versus vegetative development of timothy inflorescence was affected by plant hormones. Kinetin stimulated the vegetative development and induced proliferation both in inflorescence initials and in spikelets isolated a