1. The timing of reproduction was investigated in six Scottish freshwater pearl mussel populations from 1993 to 2002. Gravid females were examined and the release of mussel larvae (glochidia) was monitored.
2. Annual spawning (oviposition) and spat (glochidial release) events occurred during June to July and June to September, respectively.
3. Between-river differences in timing seem to be related to water temperature. Mussels in the warmest rivers tend to spawn and spat first, and vice-versa.
4. Thermal variations also seem to influence the timing of reproduction within rivers, which can be delayed by several weeks during cold years. At least 3000°-days occur between annual episodes of glochidial release.
5. The timing of spawning is determined gradually, probably by a thermal summation effect.
6. The release stage occurs as a sudden, synchronised event, with most of the glochidia spat over 1–2 days, indicating that it is triggered by an environmental cue. Sudden changes in water temperature and/or river level often result in spats, and the underlying mechanism may be respiratory.