Using microdensitometry, the cell cycle was investigated in large populations (500) of Feulgen-stained nuclei in conidia, hyphae and resting structures of various strains (all haploid) of Verticillium albo-atrum and V. dahliae as well as a stable diploid strain, V. dahliae var. longisporum. When uninucleate conidia, harvested from 3 d Roux bottle agar cultures, were incubated in stationary liquid complete medium at 24 °C in the dark, they exhibited partial synchrony for germ-tube emergence (50% emergent at about 7 h, reaching 95% at 10 to 12 h), DNA replication (50% increase at about 7.5 h) and mitosis (50% at about 10 h, reaching 95% at 12 h); the second nuclear division occurred at 21 to 24 h. Significant amounts of DNA replication were not detected until 6 h; maximum levels, i.e. a doubling of DNA content (in nuclei not yet divided), occurred at 10 h. There was a 3 h G1 phase between the first and second nuclear divisions; the S and G2 phases were estimated at about 30 and 90 min duration, respectively. The absolute DNA content per nucleus for ungerminated conidia of one strain of V. dahliae was found to be 0.025 to 0.030 pg. In growing hyphae, only the apical cell(s) contained two or more nuclei; DNA levels approximated to those for ungerminated conidia (G1) but a small number of nuclei had begun the S phase. In hyphal tip regions comprising up to 10 cells, a synchronous nuclear division was observed, and nuclei in the apical cells often had higher DNA values than other nuclei in the same hypha. Cells of young microsclerotia (6 to 8 d-old) of V. dahliae and resting mycelium (9 to 12 d-old) of V. albo-atrum were haploid and uninucleate with a small minority of the nuclei in the S phase; no evidence for diploidy or endopolyploidy was found at these sampling levels.