Dietary protein source has been shown to affect broiler chicken growth performance and muscle accretion. Myogenic stem cells (MSC), also called satellite cells, play an essential role in post-hatch skeletal muscle growth. A randomized complete block design experiment was conducted to assess the effect of dietary protein source on broiler growth performance, Pectoralis major (PM) MSC mitotic activity and heterogeneity, and muscle fiber cross-sectional area (CSA). On d of hatch, female Yield Plus × Ross 708 broilers (n = 360) were placed in raised floor pens with new litter and randomly allotted to 1 of 3 dietary treatments. The 3 corn and soybean meal-based crumbled diets fed from d 0 to 20 included: 1) soybean meal as the sole protein source (SBM), 2) soybean meal + a 50% poultry by-product meal and 50% poultry feather meal blend (PBM), or 3) soybean meal + porcine meat and bone meal (MBM). From d 0 to 8, birds were housed with 4 birds per pen (0.05 m2 per bird) and individually (0.21 m2 per bird) from d 9 to 20. On d 20, 1 h prior to PM muscle sample collection, birds (n ≥ 2 per diet; total n = 12) were intraperitoneally injected with 5’-bromo-2’-deoxyuridine (BrdU) to label mitotically active cells. Muscle samples were analyzed using cryohistology followed by immunofluorescence staining to facilitate enumeration of the mitotically active (BrdU+), MSC populations expressing the myogenic regulatory factors (MRF) and common MSC markers, MyoD and Pax7, by digital fluorescence micros- copy. Muscle fiber density on a mm2 basis and fiber CSA were determined for each bird. Data were analyzed with SAS (V9.4) PROC GLIMMIX and means were separated with PDIFF at P ≤ 0.05. Broiler feed intake, BW gain, and FCR from d 0 to 20 were not altered by dietary protein source (P ≥ 0.1925). The PM muscle MSC mitotic activity and MRF heterogeneity as well as fiber CSA and density were similar among broilers fed the 3 different dietary protein sources (P ≥ 0.1304). However, birds fed SBM and PBM had a greater proportion of fibers with CSA from 1,500 to 1,999 μm2 compared with MBM (P = 0.0227).