At some life-cycle stages, it is impossible to distinguish between the two species of porcine nodular worm, Oesophagostomum dentatum and Oesophagostomum quadrispinulatum, using morphological features. A PCR-based single-strand conformation polymorphism technique was established to overcome this limitation. The rDNA region spanning the second internal transcribed spacer was amplified by PCR from genomic DNA from morphologically well-defined adult worms. The PCR products were then denatured and subjected to electrophoresis in a non-denaturing gel matrix. Single-strand conformation polymorphism analysis of the products generated characteristic and reproducible patterns for each of the two species and allowed their unequivocal delineation. The single-strand conformation polymorphism was also applied effectively to assess the purity of nine laboratory-maintained cultures of infective third-stage larvae believed to be monospecific for O. dentatum or O. quadrispinulatum. The analysis showed that all six O. dentatum cultures were indeed monospecific, whereas the three cultures believed to be monospecific for O. quadrispinulatum were either a mixture of O. dentatum and O. quadrispinulatum larvae or pure O. dentatum larvae. These findings demonstrated the usefulness of the single-strand conformation polymorphism approach for the routine monitoring of the purity of parasite "lines" and indicated its value for studies on the population biology of porcine nodular worms.