A comparison of testate amoebae assemblages from the Arctic and Antarctic (areas of similar habitat a maximum distance apart) is used to try and answer the question ‘What is the upper size limit for cosmopolitan distribution in free-living microbes?’ Species restricted to either the Arctic or Antarctic exhibited sizes up to 230 μm while the largest cosmopolitan species was 135 μm in size. Comparison of the testate assemblages using a multivariate classificatory technique (TWINSPAN) also suggested more restricted distribution for the larger species. There was a negative relationship between species size and number of sites at which it was recorded (rs=−0.261, P < 0.05), with all the more widespread species having a size of below 100 μm. It is suggested that for testate amoebae cosmopolitan distributions become common below 100–150 μm. This suggests that most species of testate (indeed most free-living microbes) have low species richness because of lack of opportunities for allopatric speciation as most are below 100 μm and so geographical isolation is unlikely. It is suggested that if this is correct, only the largest free-living microbes (> 150 μm) are likely to be of conservation concern because of their smaller ranges. However, I point out that currently different studies are giving very different answers to the question, how ubiquitous and species rich are free-living microbes? The subject requires further work to try and reconcile these different results.