The in vitro growth behaviour of the intestinal protozoan Giardia duodenalis was studied in detail and comparisons were made between two genetically and biologically distinct cloned isolates. Replicates of each clone were grown at six different initial cell concentrations and in culture media at four different pH values. Significant differences in in vitro growth were found between the two isolates, BAH12 and P1. BAH12 had a specific narrow pH requirement, with satisfactory growth only obtained at pH 6. The mean generation time of BAH12 at pH 6 between days 1 and 3 was 10.8 h, compared to an average of 6 h for the same period for P1, both at pH 6 and pH 7. Comparative health of cultures was assessed during both the pH and growth experiments using a suite of six variables. Consistent changes in the health of cultures over time were found to reflect growth behaviour over time. These results provide the first detailed evidence that genetically different isolates of Giardia may differ in such fundamental biological parameters as growth rate and pH requirements. These differences may have important epidemiological and taxonomic implications.