The present report provides an account of the possible involvement of auxin (Indole-3-acetic acid) in callose biosynthesis, prior to cell differentiation in moss protonema (chloronema to caulonema), which is primarily an auxin-controlled event. Chloronema tip cells in Funaria hygrometrica respond to IAA (5 × 10 −5 mol/L) treatment by localised callose synthesis in the basal region of the cell before changing the pattern of cell growth and subsequent division as caulonema. A qualitative comparison of the callose accumulation response in the wild strain chloronema filaments, with those of the two auxin mutants, indicates a correlation between auxin sensitivity of the specific strain and the extent of callose accumulation along the cross wall and the adjacent lateral wall of the apical cells. The percentage of responding tip cells in the three strains varied between 65 and 80 %. Least callose accumulation is observed in the auxin-insensitive mutant 86.1. The present investigations further suggest an important role of auxin-induced influx of Ca 2+ in callose synthesis, although it does not seem to be the sole factor responsible for the regulation of this process.