Due to the detrimental effect longline fisheries have had on seabirds in other parts of the world, concern was raised as to the effect an experimental Hake longline fishery would have on Procellariiform seabirds in South Africa. Carcases of Procellariiform seabirds caught on Hake longlines during setting were retrieved to determine species, sex and stomach contents. Whitechinned petrels constituted 100% of the catch on longlines during setting despite the fact that they were not the most abundant species around fishing vessels either during the day or night. Extrapolations showed that 2 vessels were responsible for 1928 ± 1345 whitechinned petrel deaths a year in South African waters. Stepwise multiple regression suggested that light intensity was the critical factor explaining variation in the number of whitechinned petrels caught during setting. Whitechinned petrels become highly active approximately 2,5 hours before sunrise. If line shooting could be completed prior to Whitechinned Petrels becoming highly active, a great deal of mortality might be avoided. Nine great shearwaters and two pintado petrels were killed during 13 hours of hauling procedures. Hauling mortalities can be avoided by diverting offal outlets to the opposite side of the vessel to where the longline is being hauled. Larger vessels which continuously process offal seem to be more attractive to scavenging seabirds then smaller vessels. Mitigation measures must be introduced into the Hake longline fishery to reduce excessive seabird bycatch. Suggested measures include: (1) the introduction of tori lines on all vessels which catch seabirds; (2) mandatory setting of lines prior to whitechinned petrel activity peaks. Experimentation with levels of artificial lighting and with differentially weighted longlines to ensure that hooks sink immediately when placed in the water are essential to further reduce catch rates of seabirds. Without the implementation of mitigation measures population growth and long term population stability of whitechinned petrels may soon be impaired.