Invertase production under solid state fermentation (SSF) was determined using two overproducing mutants (Aw96-3 and Aw96-4) isolated previously from the wild type strain Aspergillus nigerC28B25, as well as one diploid (DAR1) and two autodiploid strains (AD96-3 and AD96-4) constructed by parasexual crossings among these mutants. Using polyurethane foam (PUF) as an inert carrier, two initial water activity (Aw) values were evaluated (0.99 and 0.96). At Aw = 0.99, maximal activity was reached by diploid AD96-4 (48.91 IU/ml) representing 30- and 13-fold increases with respect to maximal values achieved by the wild type and the haploid parental mutant (Aw96-4), respectively. Similar levels were achieved by this strain at Aw = 0.96. However, diploid DAR1 only produced high levels of invertase at Aw = 0.96 (43.90 IU/ml), whereas strain AD96-3 reached its highest production (31.10 IU/ml) at Aw = 0.99. Both productivity and yields were also analysed for every strain at each Aw value. (C) 2003 Elsevier Ltd. All rights reserved.