Context : Determining carbon and nitrogen abundances in red giants provides useful diagnostics to test mixing processes in stellar atmospheres . Aims : Our main aim is to determine carbon-to-nitrogen and carbon isotope ratios for evolved giants in the open clusters Collinder 261 and Melotte 66 and to compare the results with predictions of theoretical models . Methods : High-resolution spectra were analysed using a differential model atmosphere method . Abundances of carbon were derived using the { C } _ { 2 } Swan ( 0,1 ) band head at 5635.5 Å . The wavelength interval 7940–8130 Å , which contains CN features , was analysed to determine nitrogen abundances and carbon isotope ratios . The oxygen abundances were determined from the [ O i ] line at 6300 Å . Results : The mean values of the elemental abundances in Collinder 261 , as determined from seven stars , are : { [ C / Fe ] } = -0.23 \pm 0.02 ( s.d . ) , { [ N / Fe ] } = 0.18 \pm 0.09 , { [ O / Fe ] } = -0.03 \pm 0.07 . The mean ^ { 12 } { C } / ^ { 13 } { C } ratio is 11 \pm 2 , considering four red clump stars and 18 for one star above the clump . The mean C/N ratios are 1.60 \pm 0.30 and 1.74 , respectively . For the five stars in Melotte 66 we obtained : { [ C / Fe ] } = -0.21 \pm 0.07 ( s.d . ) , { [ N / Fe ] } = 0.17 \pm 0.07 , { [ O / Fe ] } = 0.16 \pm 0.04 . The ^ { 12 } { C } / ^ { 13 } { C } and C/N ratios are 8 \pm 2 and 1.67 \pm 0.21 , respectively . Conclusions : The ^ { 12 } { C } / ^ { 13 } { C } and C/N ratios of stars in the investigated open clusters were compared with the ratios predicted by stellar evolution models . The mean values of ^ { 12 } { C } / ^ { 13 } { C } ratios in Collinder 261 and Melotte 66 agree well with models of thermohaline-induced extra-mixing for the corresponding stellar turn-off masses of about 1.1–1.2 M _ { \odot } . The mean C/N ratios are not decreased as much as predicted by the model in which the thermohaline- and rotation-induced extra-mixing act together .