Heterocyst size variation in Nostoc muscorum has been surveyed in the presence and absence of tyrosine. The heterocyst size exhibited two major peaks under both conditions but one of the peaks shifted towards larger size in tyrosine-containing medium. Heterocysts of larger volume exhibited division in the latter medium which was not observed in medium lacking tyrosine. It is suggested that signals for cell division did not decay following differentiation of heterocyst in the presence of tyrosine.