P2 is a mutant strain isolated from the cellular slime mould, Dictyostelium discoideum strain NC-4. P2 differs from NC-4 in 3 major respects. P2 lacks the cyclic AMP chemotactic and signalling properties and the aggregative development of NC-4. P2 development consists of slow differentiation of resistant microcysts, which have not previously been reported in D. discoideum. Most important, P2 amoebae display a novel pattern of movement, quite distinct from that of NC-4 amoebae and any other related amoebae for which data are available. P2 amoebae move on agar at a mean velocity of 13.3 micrometer/min, almost twice as fast as NC-4 amoebae. P2 amoebae have a persistence time, or directional memory 'half-life', of 13.2 min, over 2.5 times the NC-4 value. However, this measure is based on straight-line movement, and actual P2 resistence may be much greater because P2 amoebae move almost always in gradual left turns and left loops interspersed with occasional long, straight segments. When P2 amoebae are plated in drops on agar, they migrate away in all directions, and the expanding drop edge can move at nearly the mean individual cell velocity. Spiral bands of amoebae can be seen in the expanding area of the drop; these invariably unwind from the original drop in a counterclockwise direction. The persistent left-turning behaviour of individual P2 amoebae is probably the major cause of this pattern of movement from drops. It also probably helps explain the similar coordinated banding behaviour observed in fields of cells. Amoebae move long distances in bands of contacting or separated cells, with no apparent organizing centre. Statistical analysis shows that P2 and amoebae have some tendency to cluster, and cell-cell contact interactions may be strong enough to promote band formation.