The reproductive cycle of the large trough shell Mactra murchisoni (Deshayes, 1854) from Pegasus Bay, New Zealand, was examined between March 2013 and February 2014. Histological analysis indicated that M. murchisoni is dioecious (there were no observed hermaphrodites) and gametogenic development is synchronous between the sexes. Gonadal development began in May in both male and females, with the first ripe individuals observed in June and July for male and females, respectively. Spawning began in spring, peaking in late summer with resorption taking place around autumn. No resting samples were observed. Condition indices were calculated each month and were at their highest during the period individuals were ripe. These indices were significantly different (P < 0.05) between the majority of months and indicate that a high portion of individuals' mass was gonadal tissue during ripe periods. Finally, the results displayed parity between the sex ratio (P =0.2782), reinforcing that the species are dioecious.