The benefits of long-chain fatty acids (FA) on human health have long been recognized, in particular the long-chain n-3 polyunsaturated FA (LC n-3 PUFA) such as docosahexaenoic acid (DHA). Microalgae (ALG) is the primary producer of LC n-3 PUFA in the food chain, and supplementing the diet of dairy cows with ALG has been shown to increase the content of DHA in milk, although little work has been conducted on the effect of rate of inclusion on milk or cheese composition. The objective of the current study was to determine the effect of rate of inclusion of ALG on milk and cheese FA profile. Twenty cows yielding approximately 40 kg/d at the beginning of the study were randomly allocated to 1 of 4 diets, in a 4 × 4 Latin square design, with 4 periods each of 28 d duration. The basal ration contained corn and grass silages and concentrate feeds, and was supplemented with one of 4 levels of ALG; 0 (Control; C); 50 (Low; LA); 100 (Medium; MA) or 150 (High; HA) g/cow per day. Milk samples were taken on 2 consecutive milkings during the final week of each period, the milk fat extracted and methylated before FA analysis by gas chromatography. Milk was also collected for making into a cheddar cheese. Data were analyzed using Genstat (v17). Supplementation with ALG had no effect on milk yield (P > 0.005) which averaged 38.5 kg/d, but milk fat content decreased linearly with increasing inclusion rate (P = 0.01), with mean values of 39.6, 38.4, 37.1 and 35.9 g/kg for C, LA, MA and HA respectively. Supplementation with ALG reduced the saturated FA content of milk from 68.7 in C to 66.7 g/100g in HA, and in the cheese from 67.9 to 66.2 g/100g. The PUFA and cis-9,trans-11 conjugated linoleic acid content of milk and cheese increased linearly (P < 0.001) with the addition of ALG in the diet. Milk fat content of DHA increased linearly from 0.08 in C to 0.37 g/100g in HA, and cheese DHA content increased from 0.06 in C to 0.35 g/100g in HA. There was no effect (P > 0.05) of treatment on cheese yield which averaged 0.26 kg/kg. In conclusion ALG can be added to the diet of dairy cows to improve the milk and cheese content of DHA without affecting cheese yield.