An ecological study of populations of Mytilus edulis chilensis from three study sites in the Falkland Islands. Mytilus edulis chilensis is dioecious with no recorded incidence of hermaphroditism and with approximately equal numbers of males and females. Reproductive development is initiated during spring (August - September) whilst a single major spawning period occurs during the southern summer (December - March). The timing of peak reproductive condition broadly coincides with mean maximum summer temperatures (12 - 14°C). Whilst settlement of M. e. chilensis spat (190 - 2000 pm) onto artificial filamentous substrate units occurs at low levels throughout the year with a single peak during the austral summer, relatively low numbers (> 2 mm) settle into the established populations in a sporadic fashion, often bearing little relation to the timing of the reproductive cycle. Reproductive output of the three study populations ranged between 0.439 x 109 eggs. M-2, at Camilla Creek, and 1.771 x 10 eggs. m 2, at Darwin. The deposition of microgrowth bands appears to be under the control of the spring - neap lunar cycle. The age and growth rate of individual mussels as well as the age composition of the study populations were determined from winter growth checks identified within the prismatic shell layer. Size and growth rate increase with decreasing tidal elevation, whilst longevity exhibits no such relationship with tidal level. L_ and k values range between 36.3 and 91.4 mm and 0.198 and 0.702, respectively. The populations from Darwin and Goose Green consisted mainly of older mussels (3 - 8 years old), whilst those from Camilla Creek were generally younger (1 -2 years old). Allometric growth of M. e. chilensis appears to be controlled principally by age and food supply. A hitherto undescribed association between the valviferan isopod, Edotia doellojuradoi, and M. e. chilensis is reported. The relationship between a green alga, Coccomyxa parasitica (Chlorococcales, Coccomyxaceae) and its host M. e. chilensis is also documented.