The morphological and functional heterogeneity of solid tumour cells can be observed in cancer cell lines cultured in vitro. We have combined analyses of microclones developed from single cells with micropore transmigration assays to demonstrate the co-existence of cellular subsets differing in morphology and motile activity, as well as Cx43 (connexin 43) and N-cadherin expression within lung carcinoma A549 populations. ‘Fibroblastoid’ cells, characterized by high motility, polarized morphology and plasmalemmal localization of Cx43, displayed the strongest aptitude for transmigration through narrow obstacles. Due to high mitotic activity, they maintain the whole population but can also give rise to a sub-population of quiescent and immobile ‘epithelioid’ cells. Our observations indicate that phenotypic transitions between the fibroblastoid and epithelioid phenotype account for the heterogeneity of metastable A549 cell populations.