Food insecurity continues to be a major contributing factor to nutrition insecurity in developing countries. A nine item Household Food Insecurity Access Scale (HFIAS) was administered twice to 307 households among three agro-ecological zones in rural communities of Kilosa District-Tanzania to measure food security. Households were surveyed once during the rainy season (February–May) and once immediately post harvest (September–October) in the year 2011. The mean HFIAS score was the highest during the rainy season (8.15±6.06) and ranged from 0 to 26. During the harvest season, the mean HFIAS was 6.76±5.88 and ranged from 0 to 24. Using the categorical measure of food insecurity, the proportion of food secure households was higher during harvest season (31.3%) than was the case 19.9% during the rainy season. Similarly, 31.6% of the surveyed households were classified as being most food insecure during the rainy season as opposed to 26.1% during the harvest season (P=0.01). Food insecurity persisted among 39.2% of the surveyed households during the rainy season and period after harvest. The households in which at least one member offered casual labour such as the clearance of the farm fields, had 2.1 higher odds (95% CI: 1.05 to 4.29) of being food insecure. A substantial proportion of the surveyed households were food insecure across agricultural seasons and in various agro-ecological zones.