The genome size of tardigrades has been documented to date in seven species of eutardigrades, and it is among the lowest in invertebrates. In this paper the evaluation of genome size is extended to eight other speeies of eutardigrades and to one species of heterolardigradcs, in order to obtain a more comprehensive picture of minimum DNA content evolution in this phylum. The genome size of the heterotardigradePseudechiniscus juanitae(0.6 pg), measured using le cells, is found to resemble that of some eutardigrades. Thus, it is not possible to discriminate between the two classes of tardigrades on this basis. Literature and new data on eutardigrades also suggest that families or subfamilies cannot be discriminated on the basis of genome size. Nevertheless, genome size and sperm cell shape are correlated. A lower DNA content always corresponds to a more specialized male gamete. In general, in tardigrades, the low genome size and its low variation could be related to the high specialization of the phylum. Preliminary data on nuclear AT contents suggest that the variations in those values are correlated with genome size variations.