Although the endemic damselfly, Platycnemis subdilatata Selys, 1849, is widespread in the Maghreb, many aspects of its reproductive behaviour, biology and ecology are still unstudied. One particular feature of this species is that its coloration pattern continues to change during maturation and afterwards, which makes it a good model for assessing age effects on behavioural and biological components. This study aims first to investigate the reproductive behaviour and choice of oviposition site, and second to assess clutch size and egg deposition rate as a function of age in a natural population located in North-East Algeria during the reproductive season of 2012. Males seized females next to reproductive sites and in foraging sites. Non-receptive females refused to mate with males by raising their abdomen up while perched or by curving the abdomen up while flying; the angle of abdomen elevation when perched was correlated to male persistence (the number of times that a male tried to grasp the female prothorax while flying over her). Copulation lasted about 14 min, the subsequent oviposition 54 min. Field experiments on oviposition site selection showed that the females prefer to lay eggs in Typha angustifolia leaves. Clutch size and egg deposition rate pattern through age showed an increase in early mature life followed by a decrease later on.