Zooplankton plays important roles in the energy and nutrient flows in the aquatic food webs. They also influence the ocean carbon cycle by producing sinking fecal pellets and dissolved organic carbon (DOC) and regulate the structure of phytoplankton assemblages. Phytoplankton carbon ingested by zooplankton will be assimilated as biological carbon or released into the environment as carbon dioxide (CO₂), DOC, and fecal pellets. So far, it is not clear how the unassimilated carbon is allocated to the three forms. We here employed rotifer Brachionus plicatilis, a common marine species that is widespread in estuarine and coastal areas and widely used as a model species for ecotoxiological and ecological studies, to examine the carbon assimilation and release in marine zooplankton. This study quantified the carbon budgets and assimilation efficiencies (AEs) of the rotifer using ¹⁴C tracing techniques. The rotifers were fed Chlorella sp. at three different food densities (1×10⁵, 5×10⁵, and 1×10⁶ cells/mL). The rotifer was first fed ¹⁴C-labeled Chlorella for a short duration and then fed non-labeled algae at the same food density several times during a 12-h depuration period. ¹⁴CO₂, DO¹⁴C, and ¹⁴C-labeled fecal pellets were collected before each feeding interval during the depuration period. The ¹⁴C retained in the rotifer after the 12-h depuration period was calculated as the assimilated carbon. The results showed that AE was 34%-51% and decreased as the amount of food ingested by the rotifer increased. The released carbon was predominant as DOC, accounting for 37%-51%, followed by CO₂ (15%-40%) and POC (23%-34%). The results also showed that DOC and POC proportions increased whereas CO₂ proportionally decreased as food density increased. DOC excreted by the rotifers was further size-fractionated into colloidal organic carbon (COC, with size range of 3 kDa to 0.2 μm) and low molecular weight organic carbon (LMW, < 3 kDa in size). The results showed that COC and LMW accounted for 33%-43% and 57%-67% of the total DOC, respectively. The COC proportion increased as food density increased. This study shows that the rotifer can transform a substantial fraction (18%-34%) of their ingested food carbon into DOC, which provides carbon sources for bacterial growth and fuels the microbial loop. The high proportions of organic carbon released by rotifer also support the idea that zooplankton diel vertical migration could be an important source of organic carbon for the mesopelagic ocean.