In connection with a proposal by Mennema to record the number of individuals of the rarest phanerogams of the Netherlands each year, some aspects of frequency fluctuations in natural populations are discussed. Some practical points regarding the counting of the numbers of the individuals in their growing sites are also mentioned. The fluctuation in numbers in a population of Anacamptis pyramidalis in the conservation area „Noordhollands Duinreservaat” is treated in greater detail.