Spirurid nematodes (family Physalopteridae) are widespread as adults or as encysted larvae in many species of Australian reptiles. Fifteen species of physalopterine nematodes (subfamily Physalopterinae) in the genera Kreisiella, Abbreviata and Skrjabinoptera infect more than 40 species of reptile in the five families Agamidae, Varanidae, Gekkonidae, Scincidae and Elapidae. Four species of nematode are host-species specific, six are host-family specific to varanid lizards, and three to agamid lizards. Larger species of reptile support a higher prevalence and abundance of nematodes, and often support multiple infections with more than one species, with the potential for interspecific competition. Geographic distribution of nematodes is partially limited by host distribution, and by climatic factors, mainly precipitation and temperature. There are strong positive and negative associations between several pairs of nematodes. Two species of nematode with the most pronounced muscular development at the anterior end, Abbreviata tumidocapitis and Abbreviata glebopalmae, only occur concurrently, and in low numbers, with species of nematode without these morphological features, suggesting differences in feeding in the hosts' stomachs. A combination of host specificity, geographic distribution and habitat, climatic factors and feeding organ morphology are factors that probably reduce the potential for interspecific competition. There is no evidence that concurrent infections affect either prevalence or abundance of nematodes, or cause discernible pathological changes to their hosts. © CSIRO 2014.