Illegal catch of abalone and lobster along the west coast of the Baja California Peninsula (Mexico) was estimated from surveys conducted among fishers and data of custom duties on live lobster exported to the USA. We estimated that illegal catch of abalone amounts to 62 t yr–1 (27% of the commercial catch) and illegal catch of lobster amounts to 64 t yr–1 (5% of the commercial catch). The two methods used in this study to estimate the illegal lobster catch yielded similar and consistent results (4.7% of the legal catch when using survey data and 4.5% when using data on custom duties for 1989–2002). We conclude that the methods of estimation are useful and reliable because they are based on direct sources of information. The methods could be used for a bottom-up approach to estimate illegal catch.