Our previous studies demonstrated that supplementing 7.5% defatted microalgae in diets for weanling pigs improved their blood hemoglobin concentrations over the controls (11.0 ± 1.5 vs.13.0 ± 1.4 g/dL). This experiment was to determine if a lower concentration of microalgae alone or in combination with phytase produced similar improvements. A total of 30 weanling pigs (Hampshire × Yorkshire × Landrace crossbred, 7.5 ± 0.4 kg BW, hematocrit: 27.5 ± 1.7%, and hemoglobin: 9.4 ± 0.6 g/dL) were allotted to 5 dietary treatment groups (n = 6/group). The 5 diets included (1) a corn-soybean meal basal diet (BD) without supplemental inorganic iron (negative control), (2) BD + 50 mg Fe/kg (as FeSO₄, positive control), (3) BD + 0.5% defatted green microalgae (Nannochloropsis oceanica, Cellana, Kailua-Kona, HI), (4) BD + 500 units/kg of phytase (OptiPhos, Huvepharma, Peachtree City, GA), and (5) BD + defatted microalgae + phytase. The trial lasted for 6 wk and growth performance and hematology of pigs were measured biweekly. Individually penned pigs were considered the experimental unit. All data were analyzed by one-way ANOVA with or without time-repeated measurements using PC-SAS (Version 9.1, SAS Institute, Inc., Cary, NC). Blood hemoglobin concentrations and hematocrit of pigs fed the negative control diet were lower (P < 0.05) than those fed the positive control diet at wk 4 and(or) 6, whereas pigs fed the 3 diets containing microalgae and(or) phytase had similar values to the positive control group for both measures at both time points. The experimental diets exerted effects on BW, ADG, ADFI, and gain/feed ratio similar to those on hemoglobin and hematocrit. However, the diets had no effect on plasma lipid or glucose profiles of pigs. In conclusion, supplementing 0.5% defatted green microalgae alone or with 500 units of phytase/kg was very effective in improving iron utilization for hemoglobin repletion in the weanling pigs. Supported in part by USDA/DOE Biomass R&D Initiative Grant, a Hatch Grant of Cornell University, and a Morley Student Research Fund Award of Cornell University.