An investigation was made into the reproductive biology of Labrus bergy1ta, Labrus ossifagus, Creni1abrus melops, Centrolabrus exoletus and Cteno1abrus rupestris (Pisces:Labridae) to examine the occurrence and control of sex-inversion. L.bergylta and L.ossifagus are protogynous, the former monandric and the latter diandric; a distinct colour change is associated with sex-inversion in L.ossifagus. The other three species are gonochoristic and only C.me1ops exhibits sexual dimorphism which involves body colour and size of urino-genita1 papilla. Analysis of age and growth data showed that L.bergy1ta and L.ossifagus are large, slow-growing species of great longevity (maximum 25 and 17 years respectively). Back-calculations of length for age were made from the opercular bones using a modification of Lee's (1920) formula: Ln - c -- O(L On- c). This showed that there is an increase in growth rate for secondary males following sex-inversion, but primary males follow the female pattern. Sex-inversion occurs between the ages of 5-20 (L.bergy1ta) and 7-13 (L.osSifagus), but only a small pr~portion of the females invert. This can be deduced since the sex ratio of secondary males to females is only 0.11 and 0.31 respectively, and females occur in the oldest age groups. The three smaller - gonochoristic species have life-spans of less than 9 i y~ars and males have a faster growth rate than females after age 2. There is no change in the length-weight relationship (calculated using the formula: Log weight (gm) = Log a + b Log length (mm) ) during the life span of any of the species. Sex-inversion in L.bergylta and L.ossifagus was confirmed by histological examination of the gonads. It occurs by atrophy of the oocytes and the gradual development of spermatogenetic cysts throughout the gonad. All 5 species undergo normal annual cycles of reproduction. In L. ossifagus, primary and secondary testes have different structures. The latter retains the lamellar arrangement of tissue and central lumen of the ovary; has a secondary vas deferens running in the wall of the gonad as a series of lacunae; and frequently has remnant oocytes. All testes of L.bergylta examined, had a secondary structure with the single exception of a very young male(7 years) in which neither a primary nor secondary structure could be confirmed In C.melops a small number of unusual males are present (ca. 11%). These have female secondary sexual characters and mature 2 years early, but are not the product of sex-inversion. Histochemical techniques failed to reveal the sites of steroid production in the gonads. The control of sex-inversion by hormones was investigated by injection of fish with methyl testosterone estradiol benzoate, and progesterone. Gonadal inversion was not achieved by these means, but methyl testosterone caused the breakdown of oocytesin L.bergylta and L.ossifagus; and induced a colour change from the red to the blue phase in L.oosifagus.