The population biology ofPlanaria torva (Müller) (Turbellaria, Tricladida)

1. A quantitative study of the population biology ofPlanaria torva (Müller) living in a productive lake, based on monthly samples over a period of 20 months, is presented. 2. Samples of triclads, their cocoons and other organisms were taken from bricks placed at a shallow depth in the lake. They provided relative information on changes in population size structure and demonstrated the temporal breeding of this triclad. 3. Cocoon production byP. torva began in January and reached a peak in March-April. This was followed by a peak in recently-hatched young during May-June. Adults reached a maximal proportion of the total population in January and most had died by July. 4. Changes in size structure suggest that intra-specific competition occurred during June-July following the recruitment of young. This hypothesis is supported by other independent evidence. 5. Attempts to measure fecundity based on cocoons and young gave very different results attributed to the sampling technique.