Under certain growth conditions unicellular organisms behave as highly organized multicellular structures. For example, the fruiting bodies of myxobacteria(1) and of the slime mould Dictyostelium discoideum(2) form structures composed of non-dividing motile cells. Although non-motile, yeasts can create organized structures, colonies in which cells communicate and act in a coordinated fashion. Colony morphologies are characteristic for different species and strains. Here we describe that, in addition to short-range intracolony cell-cell communication, yeasts exhibit long-distance signals between neighbouring colonies. The volatile alkaline compound ammonia, transmitted by yeast colonies in pulses, has been identified as a substance mediating the intercolony signal. The first alkaline pulse produced by neighbouring colonies is non-directed and is followed by acidification of the medium. The second pulse seems to be enhanced and is oriented towards the neighbour colony. Ammonia signalling results in growth inhibition of the facing parts of both colonies. This phenomenon is observed in different yeast genera. The presence of amino acids in the medium is required for ammonia production, Colonies derived from the yeast Saccharomyces cerevisiae shr3 mutant, defective in localization of amino-acid permeases(3), do not produce detectable amounts of ammonia and do not exhibit asymmetric growth inhibition.