Batch experiments with two Texas surface waters were employed to study the removal of the precursors of trihalomethanes (THMs) and haloacetic acids (HAAs) by ozonation alone and with subsequent biodegradation. Ozone doses of 0.5 to 5 mg/mg total organic carbon (TOC) were tested. Little biodegradation of THM precursors occurred in the absence of preozonation, whereas 25-30 percent of HAA precursors were biodegraded without ozonation. Ozonation and biodegradation combined removed up to 50 percent of THM precursors and up to 70 percent of HAA precursors. In some cases, ozone dosages larger than that now used in practice (1 mg/mg TOC) enhanced removal by the two processes combined. HAA precursors appeared to be more biodegradable than THM precursors, and ozonation enhanced HAA precursor biodegradation more than precursor biodegradation. Includes 19 references, tables, figures.