INTRODUCTION In mammals, studies clearly demonstrate a cross talk between adipose and muscle tissue. It is now evident that these tissues have a key endocrine role and participate in regulating each other's growth. Secretion of bioactive molecules by adipose and muscle tissues, respectively named adipokines and myokines, is well documented in mammals. In fish, research dedicated to either of these tissues is only beginning to characterize those cytokines, but very few studies to date have shown communication between these two tissues. However, indications of such cross talk are beginning to emerge in fish. For instance, a defect in zebrafish muscle development has been shown to increase intramuscular adipocyte infiltration. Furthermore, genetic selection of rainbow trout (Oncorhynchus mykiss) for muscle and subcutaneous lipid content suggests a correlation between higher adiposity and a higher proportion of large muscle fibers alongside a lower proportion of small fibers in 300g trout. Using these Fat and Lean trout lines, we aim to compare the myogenic capacity of mononuclear muscle cells extracted from their muscle. Additionally, we aim to investigate their response to mature adipocytes, in vitro. METHODS Rainbow trout (Oncorhynchus mykiss) from Fat and Lean lines were used for the isolation of mononucleated muscle cells and mature adipocytes from perivisceral adipose tissue. In vitro development kinetics studying the proliferation and myogenic differentiation of muscle cells were performed at 2, 4, and 6 days. The effect of mature adipocytes on the proliferation and differentiation of mononucleated muscle cells was determined using a coculture system based on transwell inserts. Proliferation was assessed using BrdU labeling and myogenic differentiation by immunofluorescence staining for myosin or in situ hybridization for marker gene expression analysis. Automated quantification of positive cells was performed using Fiji software. RESULTS & DISCUSSION During post-larval development, we observed a divergence in lipid accumulation across the entire fillet of the Fat and Lean lines as early as the 5g weight. The developmental kinetics of muscle cells from our trout lines in vitro are consistent with observations in the standard trout line. We showed a progressive increase of cells undergoing myogenic differentiation throughout the culture, and a peak of cells in proliferation at day 4. However, it is noteworthy that muscle cells derived from the Fat line exhibit a faster capacity of myogenic differentiation in vitro compared to those from the Lean line. We demonstrated that there is no difference in the proportion of extracted myogenic cells between our two trout lines that could account for this discrepancy in differentiation capacity. Mature adipocytes from both trout lines increased proliferation and decreased myogenic differentiation to the same extend. Muscle cells extracted from the Fat line exhibited similar responsiveness to mature adipocytes as those extracted from the Lean line. Further research to elucidate the intricate genetic and molecular mechanisms driving the differential differentiation of muscle cells is needed to unravel the complexities underlying their phenotypic divergence.