The oxidation of ammonia (NH₃) and methane (CH₄) was investigated in an alkaline saline lake, Mono Lake, California (U.S.A.). Ammonia oxidation was examined in April and July 1995 by comparing dark ¹⁴CO₂ fixation rates in the presence or absence of methyl fluoride (MeF), an inhibitor of NH₃ oxidation. Ammonia oxidizer‐mediated dark ¹⁴CO₂fixation rates were similar in surface (5–7 m) and oxycline (11–15 m) waters, ranging between 70–340 and 89–186 nM d⁻¹, respectively, or 1–7% of primary production by phytoplankton. Ammonia oxidation rates ranged between 580–2,830 nM d⁻¹ in surface waters and 732–1,548 nM d⁻¹ in oxycline waters. Methane oxidation was examined using a 14 CH₄ tracer technique in July 1994, April 1995, and July 1995. Methane oxidation rates were consistently higher in July, and rates in oxycline and anaerobic bottom waters (0.5–37 and 7–48 nM d⁻¹, respectively) were 10‐fold higher than those in aerobic surface waters (0.04–3.8 nM d⁻¹). The majority of CH₄ oxidation, in terms of integrated activity, occurred within anoxic bottom waters. Water column oxidation reduced the potential lake‐atmosphere CH₄ flux by a factor of two to three. Measured oxidation rates and water column concentrations were used to estimate the biological turnover times of NH₃and CH₄. The NH₃ pool turns over rapidly, on time scales of 0.8 d in surface waters and 10 d within the oxycline, while CH₄ is cycled on 10³‐d time scales in surface waters and 10⁻² time scales within oxycline and bottom waters. Our data suggest an important role for NH₃ oxidation in alkaline, saline lakes since the process converts volatile NH₃ to soluble NO₂⁻, thereby reducing loss via lake‐atmosphere exchange and maintaining nitrogen in a form that is readily available to phyto‐plankton.