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In the sailfin molly, Poecilia latipinna, seven morphological endocrine cell-types could be distinguished with the electron microscope. Each of these was identified with one of the seven cell-types distinguished with the light microscope, to most of which endocrine functions have previously been allocated. Corticotrophs and prolactin cells form the rostral pars distalis, and the proximal pars distalis consists of an outer layer of gonadotrophs and an inner zone containing growth hormone cells and thyrotrophs. The pars intermedia contains two cell-types, of uncertain function. Stellate cells (interstitial cells) occur throughout the adenohypophysis, but are most numerous and prominent in the rostral pars distalis. The inner proximal pars distalis contains a cell-type not previously distinguished in this species with the light microscope, the Z-cell, which could be aminergic. The ultrastructural features of each cell-type are described in detail, and discussed in comparisons with the homologous cells described in other teleosts. There is good agreement for different teleosts in the ultrastructural details of each cell type.

SUMMARY The age of Llyn Tegid trout can be reliably determined from the scales byby counting the number of completed winter bands. The winter rings are formed annually in September‐October. The seasonal growth pattern of lake trout has been investigated by examination of the scale structure each month and by following the monthly changes in the mean lenghts of each year‐class. Growth is confined to the period February‐March to September‐October, no growth taking place during the winter. Annual growth has been studied by determining the mean length of trout at the beginning of each year of life. The mean specific growth rate in the lake and streams declines with age and its negative acceleration decreases with age. Growth in Llyn Tegid and the neighbouring streams has been compared with growth in other British waters. Growth in the tributary streams is better than in the moorland streams but poorer than in the lake. Trout show an increase in growth rate when they migrate from the tributary streams to the lake. The length‐weight relationship is the same for trout of all ages and in all the localities studied. Seasonal changes in the usual condition factor, K= weight 104/length3 have been determined. K is correlated with the amount of reserve fat stored around the stomach and pyloric caeca.