Abstract Short-term fluctuations in the number of culturable microorganisms and the composition of the bacterial community colonising the phyllosphere of field grown sugar beet were determined at 6 h intervals over a 42 h period. The number of culturable bacteria and Erwinia were found to fluctuate diurnally, increasing in size overnight and declining during daylight hours. In contrast, the number of Arthrobacter and yeasts were constant throughout the study, even after rainfall. Over most of the study the number of filamentous fungi and pseudomonads decreased. The short-term fluctuations in bacterial numbers were similar in magnitude to those detected over most of the previous season. No significant variation was observed in microbial numbers between samples taken from three locations within the field. Fatty acid analysis of isolated bacteria revealed the community to be similar in composition to that detected during the same period the previous year and indicated no significant short-term temporal or spatial variation. To limit the effects of diurnal fluctuations on the number of culturable bacteria detected, it is recommended that in long-term studies, samples be collected at the same time of day on each occasion.