Assimilation levels of the antibacterials trimethoprim (TMP) and sulfamethoxazole (SMX) in sea bass (Dicentrarchus labrax) fry tissue administered orally were investigated. A 1:5 TMP and SMX combination incorporated in an oil emulsion (Selco) at 20 % and 40 % concentrations (w/w) were bioencapsulated in Artemia (Instar II) nauplii. Chemotherapeutics-loaded ('medicated) nauplii were fed to the sea bass fry and drug concentrations in the tissue were analysed by high-performance liquid chromatrography (HPLC). Fish fed 40 %'medicated Artemia assimilated significantly higher levels of chemotherapeutics compared with those fed 20 %'medicated' Artemia. Chemotherapeutics given at 40 % reached peak levels (19.3 μg TMP/g DW and 23.31 μg SMX/g DW) within 2 h while those at 20 % peaked (8.74 μg TMP/g DW and 6.73 fig SMX/g DW) after 5 h. TMP persisted in the tissues longer (up to 72 h) than SMX (12–16 h), suggesting a more efficient uptake and retention of the former and/or faster metabolism and elimination of the latter.