Abstract. The optical properties of Case 1 waters have been empirically related to the chlorophyll concentration, [Chl], historically used as an index of the trophic state and of the abundance of the biological materials. The well-known natural variability around the mean statistical relationships is here examined by comparing the apparent optical properties (spectral downward irradiance attenuation and reflectance) as a function of [Chl] in two Case 1 environments, the Pacific and Mediterranean waters. These oceanic zones apparently represent two extremes of the possible bio-optical variability range around the mean. The systematic deviations, in both directions with respect to the average laws, mainly result from the differing contents in non-algal detrital materials and dissolved colored substance for a given [Chl] level. These contents are higher than the average in the Mediterranean Sea, and lower in the Pacific Ocean, respectively. These divergences between the two water bodies, detectable in the visible spectral domain, are considerably accentuated in the UV domain. The bio-optical properties in this spectral domain (310–400 nm) are systematically explored. They are more varying for a given [Chl] than those in the visible domain. Their prediction based on the sole [Chl] index is thus problematic, although it is probably possible on a regional scale if reliable field data are available. It does not seem, however, that ubiquitous relationships exist for this spectral domain for all Case 1 waters at global scale.