The red pigment of Palœmonetes vulgaris appears to be astacin, while the yellow seems to be identical with plant carotin. A method is set forth to measure the rate of formation and destruction of pigment within the shrimps. It is observed that the red and blue pigments are most rapidly formed and destroyed, while the yellow and white pigments are much more slowly changed. Upon a white background in the light the red and blue pigments are destroyed very rapidly. Yellow disappears slowly. Upon a black background in the light the red and blue pigments are formed. Shrimps kept in darkness at first lose through destruction their red and blue pigment but after about two weeks there occurs a reversal of this process (pigment formation).