Peanuts purchased in outlets from Nairobi province were more contaminated than peanuts from Western and Nyanza provinces. Peanut products sold in Nairobi are rarely grown in the region, and are either transported from other regions locally or internationally. Furthermore, contamination of peanuts by aflatoxins can occur during production, storage, transportation and marketing (Hell and Mutegi, 2011; Nigam et al., 2009). In the case of Nairobi, the majority of peanuts are transported from other regions – whether within the country or from outside – by trucks (data not shown), and this could take one or several days, during which environmental factors such as rainfall, humidity, temperatures and respiration are likely to accelerate contamination by aflatoxin. Aflatoxin contamination has been shown to increase 10 fold in a 3-day period, when grains are stored with high moisture content (Hell et al., 2008). The findings in this study also indicated that many products traded in Nairobi come from other countries and are mainly traded through middlemen, both aspects which were linked to an increase in the likelihood of aflatoxin contamination. Aflatoxin contamination could be further aggravated by poor storage facilities, which was evident from the high proportions of peanuts recorded as infested with insects and stored in dusty, poorly ventilated premises.