Chitosans with different chemical compositions and molecular weights have been evaluated as flocculants of Escherichia coli suspensions. The flocculation performance of chitosans at different conditions (pH, ionic strength) was followed by residual turbidity measurements. For precise comparison, the chitosan concentrations corresponding to 75% flocculated bacteria (x(75)) were calculated from a mathematical function fitted to the measured data. At all conditions, an increase in the fraction of acetylated units (F(A)) resulted in lower x(75) and thereby better flocculation efficiency. Especially the most acetylated chitosans (F(A) 0.49 and F(A) 0.62) were excellent flocculants. An increase in F(A) from 0.002 to 0.6 caused a 10-fold reduction in necessary concentrations, at both pH 5 and 6.8. pH was a rather insignificant factor in the range 4-7.4, further pH increase led to either increase of necessary doses at low F(A) or sudden ceasing of flocculation at high F(A). The chitosans flocculated in a broad range of molecular weights, although an increase in molecular weight was a favorable factor. Increase in ionic strength caused a severalfold reduction in x(75) for all chitosans and considerable broadening of flocculation intervals.