In this study tetraploid Marsupenaeus japonicus (Bate) embryos were produced by preventing the first division in mitosis. The effectiveness of temperature and chemical shocks for producing tetraploid M. japonicus were assessed when applied at different times postspawning and for different durations. Tetraploid M. japonicus embryos (spawned at 27°C) were produced by heat shocks at 35°C and 36°C in three and eight spawning samples respectively, and a cold shock at 5°C in a single spawning sample. All temperature shocks inducing tetraploidy were applied 18–23 min postspawning for a 5–10 min duration. The percentage of spawnings successfully inducing tetraploid embryos (i.e., frequency of induction) ranged from 33.33% to 66.67% for the 21, 22 and 23 min postspawning heat shock treatment regimes. The percentage of tetraploid embryos within an induction (i.e., induction rate), as determined by flow cytometry, ranged from 8.82% to 98.12% (ave. ± S.E.) (34.4 ± 21.4%) for the 35°C shock treatments, from 13.12% to 61.02% (35.0 ± 5.0%) for the 36°C shock treatments and was 15% for the 5°C cold shock treatment. No tetraploids were produced for spawnings that received heat shocks above 36°C or below 35°C, or for cold shocks above 5°C for any of the tested postspawning treatment and duration times. Chemical shock with 150 μM 6-dimethylaminopurine did not result in tetraploid M. japonicus embryos at any of the tested postspawning treatment times and durations. Tetraploid M. japonicus embryos were nonviable, with no tetraploid larvae being detected by flow cytometry. Based on our results heat shocking of M. japonicus embryos at 36°C, 23 min postspawning for a 5–10 min duration is the most effective means to produce tetraploids through inhibition of the first mitotic division (taking into consideration the importance of frequency and induction rate equally).