Abstract: The possibility of developing experimental chloroquine resistant Plasmodium berghei NK65 from chloroquine sensitive Plasmodium berghei NK65 was evaluated. Five mice of about 12 weeks old were inoculated with Plasmodium berghei (CQ sensitive strain). Exactly 72 h after inoculation and confirmation of parasitemia, these mice were treated with 10 mg kg-1 body weight (b.wt.) every 48 h for one month. After this period, treatment was withdrawn for one week, following which sub-inoculation was made from each of the five mice to four new mice for each group respectively. Seventy two hours after parasitemia was confirmed in the sub-inoculated mice, two of the four mice in each group were treated with the correct dose of chloroquine, that is, 25 mg kg-1 b.wt. daily for four days, while the rest were not treated. Parasitemia was monitored in all the groups for two weeks using thick and thin smears of blood films made from the tail vein of mice and stained with 10% Giemsa stain at pH 7.2. Two weeks after treatment with 25 mg kg-1 b.wt. dose of chloroquine was stopped, four mice died in the first two groups, while one mouse each died in the remaining three groups. Six of the untreated mice from the replicate groups equally died beyond two weeks, while four survived. At death, the % parasitemia of mice that died were higher than those that survived after 2 weeks. These results suggest that those mice that survived two weeks after treatment with the right dose of chloroquine (25 mg kg-1 b.wt. for 4 days) contained chloroquine sensitive Plasmodium berghei NK65 before they were cleared, while those that had persistence of parasitemia at relatively high level which resulted in their death contained chloroquine resistant Plasmodium berghei NK65. This finding should be of importance in studies involving development of new therapy for chloroquine resistant malaria.