We describe two new species of Hyalella Smith, 1874 from São Paulo state, southeastern Brazil, presenting morphological details and shedding some light on the reproductive and populational aspects of the new species. Hyalella bala n. sp. not only has a smaller mean body length in relation to H. virgineae n. sp. , but their antennas 1 and 2 are also shorter than those of H. virgineae n. sp. There is a positive but weak correlation between head size of paired males and females, and unpaired females are usually smaller than paired or ovigerous ones, for both species. The sex ratio for H. bala n. sp. favors females, while there are 28% more males than available females in H. virgineae n. sp.. Hyalella virgineae n. sp. ovigerous females carry more and larger eggs than H. bala n. sp. , probably in response to differences in habitat use. We increase to 37 the number of species of Hyalella in Brazil, and to eight in São Paulo state. [ABSTRACT FROM AUTHOR]