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SYNOPSISEighteen strains of algae, including 17 formerly symbiotic with Paramecium bursaria, were tested for capacity to release sugar. Detectable amounts of sugar were found in the supernatant fluids from 10 strains, includinq 6 strains infective for aposymbiotic P. bursaria syngen 2. The other 4 sugar-releasing strains were noninfective and released -26-46 pg sugar/mg dry rell weight rompared to -90-175 pg sugar/mg dry cell weight for infective strains. This relationship of infectivitv with capacity to release sugar supplements data that indicate a relationship of infectivity with resistance to Con A agglutination The correlation is completed if we assume that resistance to Con A agglutination and capacity for sugar release must both be present in a n algal strain for infectivity. The data thus strongly suggest that these 2 characteristics must be present for infectivity by any algal strains for aposymbiotic P. bursaria syngen 2.Index Key Words: Paramecium burraria syngen 2 ; symbiotic algae; infectivity; sugar release. PECIFIC sugar release from the cell is a unique feature of