A new transition metal Zintl phase, Yb9Zn4+xSb9, was prepared by high-temperature flux syntheses as large single crystals, or by direct fusion of the corresponding elements in polycrystalline form. Its crystal structure was determined by single-crystal X-ray diffraction. Its Ca-counterpart, hitherto known as Ca9Zn4Sb9, and the presence of nonstoichiometry in it were also studied. Yb9Zn4+xSb9 was found to exist in a narrow homogeneity range, as suggested from the crystallographic data at 90(3) K (orthorhombic, space group Pbam (No. 55), Z = 2): (1) a = 21.677(2) Angstrom, b = 12.3223(lo) Angstrom, c = 4.5259(4) Angstrom, R1 = 3.09%, wR2 = 7.18% for Yb9Zn4.23(2)Sb9; (2) a = 21.706(2) Angstrom, b = 12.3381(13) Angstrom, c = 4,5297(5) Angstrom, R1 = 2.98%, wR2 = 5.63% for Yb9Zn4.380(12)Sb9; and (3) a = 21.700(2) Angstrom, b = 12.3400(g) Angstrom, c = 4.5339(4) Angstrom, R1 = 2.75%, wR2 = 5.65% for Yb9Zn4.384(14)Sb9. The isostructural Ca9Zn4.478(8)Sb9 has unit cell parameters a = 21.830(2) Angstrom, b = 12.4476(9) Angstrom, and c = 4.5414(3) Angstrom (131 = 3.33%, wR2 = 5.83%). The structure type in which these compounds crystallize is related to the Ca9Mn4Bi9 type, and can be considered an interstitially stabilized variant. Formal electron count suggests that the Yb or Ca cations are in the +2 oxidation state. This is supported by the virtually temperature-independent magnetization for Yb9Zn4.5Sb9. Electrical resistivity data show that Yb9Zn4.5Sb9 and Ca9Zn4.5Sb9 are poor metals with room-temperature resistivity of 10.2 and 19.6 mOmega(.)cm, respectively.