Observations aboard purse-seiners demonstrated that deliberate lowering of the net to allow pelagic fish to escape (“slipping”) was frequent off northern Portugal during the second semester of 2001. Some slipping occurred in 25 of 30 trips observed, and the quantities slipped were significantly higher when the net was set on dense echo-sounder marks. During the 12 weeks of the study, the sampled fleet (nine vessels) landed 2196 t and deliberately released an estimated 4979 t (CV 33.6%). More than 95% of the total catch was sardine. Data provided by the skippers in the absence of shipboard observers led to considerably lower estimates of slipped quantities. The main reason for slipping was daily quota limitations, but illegal size and mixture with unmarketable bycatch were also reported. These results demonstrate the existence and potential magnitude of slipping, but indications of large seasonal and regional variations make extrapolations for the entire fishery impractical.