A sampling apparatus suitable for studying fluctuations in populations of the poultry red mite, Dermanyssus gallinae (Deg.), under field conditions is described. Four small infested poultry houses were sampled regularly for four years. In three of these years the highest population of the mite was recorded during August, and was followed by a fairly rapid fall in numbers during the autumn. Very few mites were found during the winter months. Attempts were made to increase winter populations by raising the temperature and humidity to August levels and also by increasing the amount of illumination during the winter months, but were unsuccessful.