Sexing individuals in a population is important in many ecological and life‐history studies. Since many bird species are monomorphic, non‐invasive tools are necessary for sex determination. In this study we utilized flow cytometry to sex individuals in a moorhen population of northern Italy. By improving previous laboratory protocols, we were able to obtain clear and repeatable measures of DNA content from field blood samples. The per cent difference in nuclear content between male and female moorhens was among the highest values reported for birds. We also utilized a discriminant analysis of seven morphological measures to investigate whether birds can be sexed on the basis of biometry. Tarsus and foot lengths were the most influential variables in gender discrimination. However, only 13 females and 10 males (77%) were correctly sexed, while six females and two males were wrongly assigned. When juvenile moorhens were excluded the discriminant analysis correctly sexed 90% of the birds. Since morphometric comparisons with English moorhen populations showed that discriminant bio‐metrical values are geographically different, and thus not useful as universal sexing tools, we recommend the use of the cytometry technique for sex determination.