The level of succinic dehydrogenase activity in Neomysis integer as determined by the reduction of triphenyl tetrazolium chloride appears to vary seasonally from 1·7 μg formozan/mg wet weight/24 hours in November, to 3·5 μg/mg/24 hours in December, and to 5·3 μg/mg/24 hours in May. Temperature accelerates the enzyme activity. There is an inverse relationship between activity of the enzyme and body weight. A comparison with a few other typical planktonic animals shows that the activity is high as compared with that of cnidarians and ctenophores, but is in the same range as that for chaetognaths and copepods. Of the chaetognaths studied, Spadella shows a higher activity than Sagitla. Similarly, among copepods, Acartia has a far higher activity than Calanus, again indicating possibly an inverse relationship with body weight.