This study was undertaken to determine the effects of genotype, differentiated state of explant and random effects on types and extent of in vitro gross chromosomal variability in celery. The proportions of normal diploid (2n = 22) cells in vitro ranged from approx. 30-80% in callus cultures. The remainder of the cells were classified as hypodiploid (<22), polyploid (44, 88, etc.) or intermediate (all others). Genetic background was shown to have a highly significant impact on proportions of chromosomally variable cells in callus. In contrast, no karyological differences were detected among cultured cells originating from the same plant. Possible relationships of genotype to in vitro chromosomal variability and applications to breeding and genetics are discussed.