This study focused on characterizing the morphological features of the RLO through histology and SEM, and assessing their taxonomic affiliations via 16S rRNA amplicon sequencing. From 2007 to 2017, the prevalence and intensity of RLO were recorded by histology. The RLO were observed within circular basophilic cytoplasmic membrane-bound vacuoles (MBV) (Fig 1) , with an average length and width of 15.70±15.24 µm and 15.42±14.95 µm, respectively. Apart from cell hypertrophy, no tissue alterations were observed in areas adjacent to the RLO. Individual bacteria within the MBV were coccoid in shape, with an average length of 0.65±0.12 µm and an average width of 0.38±0.09 µm. The bacterial microbiota of selected samples revealed the presence of intracellular parasite operational taxonomic units (OTUs) corresponding to the families Rickettsiaceae and Anaplasmataceae , suggesting an association of the RLO with the order Rickettsiales . Throughout the study period, a mean prevalence of 5% was observed, with 89% of organisms exhibiting low intensity (G1: 30-61 RLO) of the MBV. A h igher prevalence occurred during warmer months. The absence of tissue alterations, low prevalence, and low intensity of the MBV imply that RLO in C. corteziensis act as commensal endosymbionts with minimal risk to oyster production in Nayarit, Mexico. However, regular monitoring is essential to detect any variations in this relationship, particularly in scenarios of extreme environmental fluctuations.