Pollen development in 8‐year‐old hazel plants (Corylus avellanaL.) from Central Italy was monitored in order to investigate the chilling completion date of the catkins. Meiotic division occurred in September and pollen development continued until mid‐October when the first haploid mitosis divisions were observed. The catkins elongated gradually during the summer until mid‐October when their growth slowed. In mid‐December the catkins resumed rapid growth and released pollen. From the end of October the pollen grain volume decreased by 12% on average, reaching its minimum value on 6th November. From this time onwards volume increased again to reach its maximum value at the beginning of December, just before pollination. In mid‐October the daily mean temperature dropped below 12.4°C and remained at this low level during the succeeding period; a relatively high mean temperature, above 4.5°C, was recorded in the second week of December. The beginning of dormancy could be correlated with the slowing down of catkin growth and the decrease in the volume of the pollen grains after the first haploid mitosis. When the chilling requirement is fulfilled, at about 640 chill units, the pollen grain volume reaches its maximum value.