Coenzyme Q (CoQ; ubiquinone) is an essential component of the respiratory chain. It is also a potent antioxidant that prevents oxidative damage to DNA, biological membranes, and lipoproteins. CoQ comprises a six-carbon ring with polar substituents that interact with electron acceptors and donors, and a hydrophobic polyisoprenoid chain that allows for its localization in cellular membranes. Human CoQ has 10 isoprenoid units (CoQ10) within the polyisoprenoid chain. Few microorganisms produce CoQ10. This work shows that Thraustochytrium sp. RT2316-16 produces CoQ10 and CoQ9. The CoQ10 content in RT2316-16 depended strongly on the composition of the growth medium and the age of the culture, whereas the CoQ9 content was less variable probably because it served a different function in the cell. Adding p-hydroxybenzoic acid to the culture media positively influenced the CoQ10 content of the cell. The absence of some B vitamins and p-aminobenzoic acid in the culture medium negatively affected the growth of RT2316-16, but reduced the decline in CoQ10 that otherwise occurred during growth. The highest content of CoQ9 and CoQ10 in the biomass were 855 μg g−1 and 10 mg g−1, respectively. The results presented here suggest that the thraustochytrid RT2316-16 can be a potential vehicle for producing CoQ10. Metabolic signals that trigger the synthesis of CoQ10 in RT2316-16 need to be determined for optimizing culture conditions.