ABSTRACT The number of spermatozoa in a sample of sea-urchin semen (Echinus esculentus) can be accurately estimated by measuring the amount of light scattered and absorbed by a subsample composed of semen diluted with sea water. A ‘Spekker’ photoelectric absorptiometer was used for the measurements, with a cell 1 cm. long. The number N of spermatozoa in a sample of sea-urchin semen was found to be given by the equation where Y′= 0·3010 – log10y′ and y′ = absorptiometer reading, when the original sample was diluted 200 times and the absorptiometer set to 2·000 for the blank. The light-scattering and light-absorbing properties of washed and unwashed suspensions were found to be virtually identical. The number of spermatozoa in a sample of bull semen was found to be given by the equation