The pedunculated cirripede Pollicipes pollicipes (Gmelin) is of considerable economic importance in Portugal, particularly along the southwest coast. The current knowledge of its biology is poor, as evidenced by the paucity of available literature. Over a period of two years, P. pollicipes populations at Castelejo and Zavial were sampled from different tidal levels at approximately monthly intervals. Populations at two other sites, Ponta- da- Fisga and Sagres, were also sampled but at irregular intervals. The current study has provided definitive information on the annual reproductive cycle of P. pollicipes that is essential for any attempt at fisheries management. It is shown conclusively that shell-banding techniques are easily applicable to age and growth studies on these lepadomorph barnacles and has provided initial information on the growth rates and factors influencing shell deposition in experimental situations. An exhaustive data set provides a complete set of morphometric tools for the conversion of length measurements into dry and ash-free weights and demonstrates little differences between shapes of P. pollicipes on the different shores sampled. The demonstration that P. pollicipes is an obligate cross-fertilising barnacle has subtle implications in managing the fishery since it is imperative that any adopted policy incorporates aggregates of the barnacle as a unit necessary to ensure continuing recruitment. P. pollicipes when grown under simulated semi-diurnal conditions of emersion and immersion lay down clearly defined bands with almost an exact coincidence with the number of emersions. However, barnacles from the field showed no clear banding periodicity since band widths were very similar throughout the shell length with no regular "check" marks that could provide age estimation. Differences in the reproductive patterns between Castelejo and Zavial were minor and none statistically significant hence differences in environmental conditions between the two sites do not appeared to materially affect the species reproductive patterns. Adult barnacles carried fertilised eggs from April throughout September, and the percentage of individuals with egg masses correlated positively with air but not seawater temperature. Brooding success was greater with average air temperature in excess of 14 °C during months of strong upwelling and presumed food availability. Functional ovaries and testes are available in the population all year round but fertilisation does not appear to occur during the winter months. Reproduction of P. pollicipes on the southwest coast of the Algarve appears closely linked to the upwelling phenomenon with the rate of deposition of gonadal tissue probably determined by environmental temperature. Laboratory and field observations permitted to conclude that P. pollicipes is an obligate cross-fertiliser. Isolated and communal individuals were observed to carry ripe ovaries and seminal vesicles filled with sperm at the same time but fertilisation took place only on communal individuals. Increasing numbers of isolated individuals are found on the southwest coast of Portugal, probably as the result of an intense fishery. The assessment of the fishery impact on the natural population and the development of management policies for the fishery are imperative future measures in order to maintain the recruitment levels. Enforcement of reserve areas may prove a practical solution for Pollicipes management. Protected populations could serve as stocks for recruitment to other areas. A series of protected areas around the coastline would need to be established in areas that could easily be monitored and, if needed, physically protected. It might also be feasible to rotate areas every 3-4 years resulting in a better quality and increased quantity of P. pollicipes entering the fishery.