Isozyme marker loci were used to measure outcrossing in artificial and natural populations of celery (Apium graveolens). In three separate experiments involving populations of uniform density and flowering time, outcrossing rate ranged from 47 to 87% and the mean was 71.4%. In a wild population, outcrossing was estimated at 32.4 and 53.1%, using different homozygotes as the fixed female, and the weighted mean was 49.4%. A slight correlation of outcrossing rate with population density was observed. Mean outcrossing rates estimated from departures from panmictic expectations at polymorphic isozyme loci among open-pollinated plant introductions were 67.6% (celeries), 79.8% (celeriacs), and 85.1% (smallages). Based on these results, it was concluded that celery is best described as intermediate with respect to self- and cross-pollination.