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Compared with other groups of unicellular freshwater algae, desmids lend themselves well to biogeographical studies since, at species level, identification is often relatively easy, whereas high ecological demands use to curtain their geographical distribution.

1. Isolates of twelve planktonic desmid species were tested for their growth response in batch culture, using an apparatus that generates crossed gradients of temperature and light intensity.2. Up to 15°C. growth was mainly temperature-limited. At higher temperatures, there was an increasing effect of light limitation. Optimal growth temperatures were in the range 25-30°C.3. Highest specific growth rates varied from 0.28 to 1.34 day"'. Species with lowest growth rates originated from oligo-mesotrophic hahitats, those with highest growth rates from eutrophic ones.4. Ecophysiological aspects are discussed in some detail, as well as the significance of the experimental data for distributions in the field.