Within the framework of a coordinated collaborative study for evaluating assays for aneuploidy, nine known or suspected spindle poisons were tested in mitotic segregation assays with Aspergillus nidulans. Experiments with A.nidulans diploid strain P1 revealed a statistically significant increase of whole chromosome segregants (non-disjunctional diploids and haploids) after treatments with chloral hydrate (CH), thiabendazole (TB), thimerosal (TM) econazole (EZ) and hydroquinone (HQ). The latter two chemicals also increased the frequency of mitotic cross-overs. Colchicine (COL), diazepam (DZ), cadmium chloride (CD) and pyrimethamine (PY) were ineffective. Further experiments with CH, TB, TM and EZ in the haploid strain 35 demonstrated that CH, TB and TM induced hyperploid types, thus indicating a primary effect on chromosome segregation in A.nidulans. However, since EZ did not induce putative hyperploids in strain 35 and trisomics in diploid 31, it is suggested that EZ affects chromosome segregation by an mdirect mechanism, possibly related to induced structural chromosome damage, as previously shown for HQ.