The article presents an algorithm for using spectrophotometric measurement to determine the number of cells of cultures of microalgae of the genus Tetraselmis filtered and unfiltered samples. The dependence "density – number of cells" is approximated by a linear equation with a high correlation coefficient, more than 75%, which confirms the high reliability of the method. The ability to measure the density of unfiltered culture samples on a spectrophotometer allows you to quickly estimate the number of cells both in the field and on an industrial scale.