The sublittoral red alga Delesseria sanguinea was pre-cultivated in the laboratory in a predominantly green light field at 10 µmol m-2s-1 (10°C), with an approximately 0.5% daily increase in blade area. Such pre-cultivated blades were exposed horizontally in flow-through chambers for 5-6 days in April or May to solar radiation reduced to 11 or 19% by neutral density screens. In all three experiments conducted, the full solar spectrum (UVB+UVA+PAR) reduced growth rate significantly to 0-50% of values obtained in PAR alone, while this occurred with UVA+PAR only in one experiment. The growth rate of a Delesseria blade may thus be used as a sensitive and reliable biological indicator of UVB in natural solar radiation at a pre-chosen, neutral reduction level. Rapid measurements of growth rate of apical parts of Delesseria blades during and after UVB+UVA pulses of 2, 3 or 6 h duration were performed in the laboratory by measuring thallus area every 2 min by use of a CCD camera coupled to an on-line, computer-aided image analysis system. A single pulse of 2 or 3 h duration administered during the light phase caused a temporary drop in growth rate during and after the pulse, with recovery starting 2.5 h after the end of the pulse and completed by the end of the light phase. A single 6-h pulse at a biologically effective UVB dose (BEDDNA300nm) weighted according to Setlow (1974) of only 0.5 kJ m-2 reduced growth rate by 55% if administered around noon, or halted growth almost completely, if supplied at night, when no photoreactivation was possible. The UVB-sensitive behaviour of Delesseria compares well with the highly sensitive phytoplankton alga Emiliana huxleyi whose growth was reported to be halted at a daily, weighted BEDDNA300nm of 0.4 kJ m-2 administered during 4 days for 3 h in the middle of the light period.