Purpose: Algipore is a biologic hydroxyapatite derived from calcifying maritime algae. The present study evaluated this material histologically and histomorphometrically after implantation in rabbit tibia defects for 4 weeks. Materials and Methods: Six New Zealand rabbits were used in this study. In each rabbit tibia, two 7-mm defects were prepared. Control defects were left empty, and test defects were filled with Algipore. Twenty-four specimens (12 test and 12 control) were retrieved and processed for histology. Results: In control sites, newly formed trabecular bone with large marrow spaces was plentiful in the most peripheral areas of the defects but sparse elsewhere. In contrast, in test sites, a large quantity of newly formed bone around the biomaterial particles was detected in the central medullary portion of the defect. In addition, in several areas, the biomaterial particles were bridged by newly formed bone. The percentage of contact between newly formed bone and biomaterial particles was 71.2% ± 9.8%. Inside the central portion of the biomaterial particles, it was possible to see newly formed bone (about 35.3% ± 4.8% in each particle). In test sites, newly formed bone represented 31.1% ± 1.9% of the material, with residual biomaterial particles occupying 33.4% ± 2.8% and marrow spaces another 34.7% ± 4.3%. In the control sites, the values were 30.2% ± 2.2% for newly formed bone and 68.7% ± 4.1% for marrow spaces. A statistically significant difference was found in the percentage of marrow space between the two groups, but no significant difference was observed in the percentage of newly formed bone. Conclusions: The present rabbit study confirmed the high osteoconductivity and resorbability of this biomaterial. Schlagwörter: algae-derived hydroxyapatite, bone regeneration, histology, rabbit defects