Living crane-fly spermatocytes were treated with 10-20 microg/ml cytochalasin D (CD) or 0.3 microg/ml latrunculin (LAT) at various stages of meiosis I. The drugs had the same effects on chromosome behaviour, but CD effects were reversible and LAT effects generally were not. When applied in mid-prometaphase to metaphase, both drugs altered subsequent anaphase poleward movements: half-bivalents either moved more slowly than normal, or moved more slowly after a brief period of movement at normal rate or stalled for 10 min or more immediately after disjunction. CD effects were reversible: within 1 min after washing out the CD, stopped chromosomes started moving and slowed chromosomes sped up. When applied in anaphase, both drugs stopped or slowed poleward chromosome movements, usually reversibly. When applied near the end of prophase, both drugs often prevented one or more bivalents in the cell from attaching to the spindle. Attached bivalents behaved as in cells treated with drugs at later stages, as described above. Unattached bivalents in the same cells moved to poles or cytoplasm in early prometaphase, where they remained motionless; at anaphase they sometimes did not disjoin, but when they did disjoin the half-bivalents did not move, either in the continued presence of the drug or when CD was washed out, confirming that they were not atttached. When CD or LAT prevented all bivalents in the cell from attaching, spindles kept in the drug were invaded by granules at about the time of normal anaphase. Conversely, when CD was washed out during late prometaphase, chromosomes often attached to spindle fibres and later entered anaphase. As CD and LAT are different antiactin drugs, but have the same effect on chromosome behaviour, the results implicate actin in early interactions of chromosomes with spindle fibres and in anaphase chromosome movements.