In the years 2013 and 2014 the waters of the National Nature Reserve Čičov oxbow lake in the southwestern part of the Slovak Republic were analyzed for total and phosphate phosphorus concentrations. Collections of samples were inspected regularly at monthly intervals, always in the middle of the month. The results were compared with the limit values set out by the Government Regulation No. 269/2010 Coll. The average concentration of total phosphorus for individual months during both monitored years was 0.19 mg.dm^-3 with the maximum average concentration in the month of June (0.26 mg.dm^-3) and the minimum in the months of August and September (0.15 mg.dm^-3). Its average concentrations at individual sampling sites were relatively balanced and varied from 0.16 (at a side arm of the National Nature Reserve) to 0.23 mg.dm^-3 (at the river mouth of the Čilizian stream into a dead arm). The average concentration of phosphate phosphorus for individual months during both monitored years was 0.08 mg.dm^-3. Depending on the time of sample collection, its average concentrations ranged from 0.05 (November) to 0.12 mg.dm^-3 (August). Depending on the sampling site, the average phosphate phosphorus concentrations varied from 0.07 to 0.10 mg.dm^-3. The data for the year, the month, and the sampling site showed a statistically high significant effect on total phosphorus, and a statistically significant effect for phosphate phosphorus was recorded from data for the month.