Simultaneous in situ measurements of chlorophyll concentration and water colour are reported at three diverse sites: a sea loch, the west Scottish shelf and the north Atlantic. A good (R2=0.97) log-log relation exists between the ratio, γ, of the irradiance reflection coefficients at 490 and 570 nm and the chlorophyll concentration, C, over a range of chlorophyll concentrations from 0.01 to nearly 50 mg m-3. This relation can be expressed as C=1.87γ-1.81. The equation is very similar to that used in remote sensing algorithms for the conversion of ocean colour data into chlorophyll concentrations. The root mean square (RMS) variation of observed chlorophyll about the values predicted by this equation is 33%. The robustness of this algorithm implies that this colour ratio can be used to monitor chlorophyll concentrations in case 1 waters with a minimum of additional calibration information. As an illustration, a time series is presented of colour-ratio-derived chlorophyll concentration during the spring bloom in a Scottish sea loch in 1994. The data show reasonable agreement with the chlorophyll time series measured by a recording fluorometer, and a comparison of the two time series serves to highlight the advantages and disadvantages of each instrument.