The toxicity of an anatoxin-a(s) producer strain of Anabaena spiroides (ITEP-024) was estimated through sub-chronic bioassays with two clones of Daphnia similis (Labtox and Itajubá), both with intact cells and aqueous extracts of lyophilized material. Animals were grown as clonal cultures in the lab with mineral water plus 20% lake water. The concentrations used in the bioassays were 0.125, 0.25, 0.375, 0.50 and 1.00 mg·L^-1 for intact cell cultures and 10, 25, 50 and 100 mg·L^-1 for aqueous extracts. Controls with nutritive food were used. The bioassays lasted 72 hours for the aqueous extracts and 96 hours for the intact cell cultures, with measurements of survivorship every day and body length at the beginning and at the end of the bioassays. Both kinds of samples affected survivorship and growth rate of both Daphnia clones, with the intact cell samples being more effective than aqueous extracts. Regarding survivorship, the clone Itajubá was more sensitive to aqueous extracts than the clone Labtox (LC₅₀ = 54.4 and 83.1 mg·L^-1, respectively). No significant difference was found between clones in the intact cell bioassays. Regarding growth rates, a significant difference was found between clones in both samples. However, growth rate was significantly decreased in much lower concentrations of intact cells than in aqueous extracts of cyanobacteria. A stimulus of growth was found in the lower concentration (10 mg·L^-1) of aqueous extracts, which is consistent with a hormetic response. In spite of its known high neurotoxicity to mice, ITEP-024 strain caused any effect on mobility of both clones. The effects on survivorship and growth of Daphnia caused by ITEP-024 strain in much higher concentrations of aqueous extracts suggests that uptake of toxins from the water are not so effective as the uptake trough the gut when intact cells are ingested.