The expression of the gene encoding an anionic isoperoxidase from zucchini (Cucurbita pepo L., cv. Black Beauty) (APRX) has been analyzed by northern blot, in situ hybridization and isoelectric focusing (IEF). Hypocotyls and stems were the organs containing the highest amount of APRX transcripts. A detailed study of APRX expression in hypocotyls revealed that the mRNA accumulated mainly beneath the apical hook, in the outer half of the hook and in the root tip, three zones characterized by a relatively high elongation rate. Seedlings grown in darkness or under a 12-h photoperiod exhibited a maximum APRX expression 5 or 6 d after germination, preceding an acceleration of hypocotyl growth. The seedlings grown under continuous light had a shorter hypocotyl and a lower level of expression. Surprisingly, the variations of APRX transcript level observed in the different hypocotyl zones or in hypocotyls of different ages did not result in visible changes in the amounts of APRX activity, as determined after IEF separation. The distribution of lignin along the hypocotyl and in the hook was inversely related to the level of APRX transcript.