The electrophoretic mobilities of the small phages S 13. Φ Χ 174 and of a series of sequential mutants of the latter have been estimated, so that a pH-mobility diagram for each strain could be constructed. The mutants could be separated into two groups by their electrokinetic behaviour, one of the groups reacting more basically over the whole pH-range (3.5 — 9,6) than the other. Parallel with the second spontaneous mutation the mutant strain changed from the first to the second mobility curve. Only one of the following five mutants jumped back to the former curve. It is assumed that the difference in the electrokinetic behaviour is caused by the exchange of nn acidic for an uncharged amino acid and vice versa within the repeating subunit of the protein cover of the phages.