Actin is an abundant, highly expressed, and much conserved protein belonging to the actin–heat shock protein70–sugarkinase superfamily. The full-length messenger RNAs encoding actin were cloned and characterized from the plant-parasitic cyst nematodes Heterodera glycines and Globodera rostochiensis and from the free-living nematode Panagrellus redivivus. The actins from the plant-parasitic nematodes showed highest amino acid sequence identity to filarial nematode homologues, whereas nucleotide sequence identity was much lower than that for many actins from very distant organisms, such as vertebrates, plants, or fungi. Analysis of base composition revealed a striking difference between the H. glycines actin gene and those of filarial nematodes in (G C) content and usage of particular codons. Analysis of genomic DNA revealed the presence of 7 introns in the H. glycines actin gene. The first was atypically long and started with a GC dinucleotide, and the fifth intron occupied a novel site in the catalogue of intron positions known thus far for actin genes. Analysis of the partial genomic DNA sequences obtained from 6 other Heterodera spp. suggested that features shown for the H. glycines actin gene are characteristic for a wide range of cyst nematodes.