Two environmental manipulation protocols were tested against a current maturation technique (CONTROL) in an 81-day experiment. PROTOCOL 1 mimicked changes in the natural environment from winter to spring when ovarian maturation and spawning in Penaeus merguiensis starts. PROTOCOL 2 evaluated whether a clear improvement of temperature from a fluctuation period to a stable one could help trigger ovarian maturation. In phase A (first 30 days) of the experiment, temperature and day length were gradually increased from 22 to 27 àand from 10 to 14 h, respectively, while light intensity was stepwise reduced from 360 to 240, 120 and 5 lx for PROTOCOL 1. For PROTOCOL 2, light intensity and day length were manipulated as for PROTOCOL 1, but temperature was cycled from 22 to 27 àevery 3 days. In phase B (the following 30 days) temperature, light intensity and day length were stably maintained at optimal levels (27 ì 5 lx and 14 h light, respectively). During phase B, the proportion of female prawns reaching stages III and IV was significantly (P<0.05) highest in PROTOCOL 1 and lowest in the CONTROL. Of the total spawns by unablated females recorded in this phase, 55.6% was in PROTOCOL 1, 33.3% in PROTOCOL 2 and only 11.1% in the CONTROL. Mass spawning was induced by unilateral eyestalk ablation on day 60. Spawning performance of prawns were monitored for a further 21-day period (phase C). Overall, PROTOCOL 1, PROTOCOL 2 and the CONTROL produced 2.36, 1.96 and 1.53 million eggs, respectively. The latency period of spawners was shortest (P<0.05), whereas the proportion of spawners, fecundity, spawning rate and egg production/female were all improved in PROTOCOL 1. An improvement of prawn growth was observed in PROTOCOL 2. This study suggested that environmental manipulation could improve ovarian maturation and spawning rate of pond-reared P. merguiensis.