An image analysis system was used to count and measure acetoorcein-stained mitotic chromosomes of the acidothermophilic unicellular red algae Galdieria maxima, G. partita, G. sulphuraria and Cyanidium caldarium (Cyanidiophyceae). Chromosome numbers fell into two groups: n = 2 for all three species of Galdieria and n = 5(7) for C. caldarium. It seems that a separation of C. caldarium from Galdieria is karyologically justified. A chromosome number of 2 appears to be indicative of the genus Galdieria and could be used as a marker to distinguish this taxon from Cyanidium. The two smaller chromosomes in the karyotype of C. caldarium were about 0·4 μm long whereas the other three were 0·5–0·7 μm long. In karyotypes of Galdieria species, the two chromosomes differed in length, the smaller chromosome ranging from 0·8 to 1·8 μm and the larger one from 1·2 to 2·3 μm. The visualization of these extremely small chromosomes was possible due to pretreatment of the cells with the DNA intercalator 9-aminoacridine. The mean absolute length of each chromosome of the three members of Galdieria had statistically significant interspecies differences. Nuclear 1C DNA contents were estimated in the algal cells by the Feulgen technique. All species investigated had genome sizes of 1·50–2·25×10−2 pg. Thus it seems that the members of Cyanidiophyceae have the smallest known genomes of all photosynthetic eukaryotes.