The social networks of 49 ethnic Vietnamese injecting drug users (IDUs) and 150 IDUs of other ethnicities recruited in Melbourne, Australia, were examined for ethnic differences in distribution of hepatitis C virus infection risk using social network analysis and molecular epidemiology. Vietnamese IDUs were more highly connected than non-Vietnamese IDUs, and more likely to be members of dense injecting sub-networks. More related infections were detected in IDUs with discordant ethnicities than were captured in the social network data; nonetheless, most dyads and most IDU pairs with related infections had matching ethnicity, confirming that mixing was assortative on that criterion. Mixing was not obviously dissortative by risk; low-risk Vietnamese IDUs injected more frequently than did correspondingly low-risk non-Vietnamese IDUs, but results for other measures were reversed or equivocal. Network measurements suggest that ethnic Vietnamese IDUs are at elevated risk of blood-borne infection, a conclusion supported by their relatively high HIV prevalence.