Thirteen in situ measurements of primary production were made 25 miles (46 km) west of Newport, Oregon between April 1962 and June 1964. Assimilation ratios [mg C hr‐1 (mg Chl a)‐1] at light saturation ranged between 6 and 21. The mean value was 8.6 with a 95% confidence interval of ±1.3. There was no correlation between maximum assimilation ratios and total daily solar radiation. with recently upwelled water. The highest values observed were associated with recently upwelled water. It is suggested on the basis of these experiments and the work of others that assimilation ratios of 0–3 indicate nutrient depletion; ratios between 3 and 5 indicate borderline nutrient deficiency, and ratios between 5 and 10 indicate nutrient‐rich waters. It is concluded that emperically determined Cassimilation ratios, derived from as near‐natural experiments as possible, should be used in production equations.