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We have found a relationship between the net absorption of carbon at time t divided by the net absorption of carbon in 24 h (AN(t)/AN(24)) and the incubation duration after sunrise, in surface waters of the western Mediterranean sea. This ratio is quite similar whatever the sample location, the sample depth and the 24-h net-absorption level. Considering this result, we have established a correction factor which may be used to convert primary production rates obtained from incubation durations < or = 24 h to daily rates (24 h dawn-to-dawn).