Efficiency of a mechanical harvester for red raspberries was measured by using berry titratable acidity, weight and age to estimate berry ripeness. The acidity of ripening berries decreased rapidly with age and the percentage of berries removed by the harvester rose as titratable acidity fell. Unripe berries which were removed by pedicel breakage, were harvested more readily by the machine as they increased in weight. Once berries were ripe enough to be removed by separation from the receptacles the harvester efficiency continued to increase, although somewhat irregularly, with further increases in berry weight. The percentage of berries removed rose from 5% for 10-day-old berries to 77.97% for 39-46-day-old berries and then fell to 20-33% for older berries.