If rare events or particles are distributed at random in space or time (e.g., bacteria, yeast particles, colloidal particles) the frequency of zones with 0, 1, 2, etc., particles is well known to be described by the Poisson Series. An equation, deduced by the Method of Maximum Likelihood, is given for estimating the mean number of particles per zone when counts have been made only on zones containing a few particles, those zones with more than a certain number being classed together. Nomograms are given for solving the equation when zones with more than 1, 2 or 3 particles are so combined, and the standard error of the estimate is worked out. The method is useful where crowded zones cannot be counted, or in routine work where it takes much more time to count up particles in a crowded zone than to estimate the number when only 2 or 3 are visible at once.