Idli, a very popular fermented breakfast food staple consumed in the Indian subcontinent, consists mainly of rice and black gram. Idli fermentation was carried out in the conventional way in a batter having rice to black gram in the ratios of 2:1, 3:1 and 4:1 at room temperature. The rheology of the product was assessed using a Brookfield viscometer having disc spindles. Power law model with yield stress adequately fitted the data. Yield stress values were in the range of 13-43 Pa and reached a maximum value at 7 h of fermentation. Flow behaviour indices were in the range 0.287-0.605. Flow behaviour indices at 23 h were significantly lower than those at 0 h. Consistency index values, at any fermentation time, increased as the rice to black gram ratio increased. Mean particle size ranged from 500 to 600 mu m and there was no definite trend noticed with respect to time of fermentation and rice to black gram ratio. There was a steep change in volume increase after 4-h fermentation. At the end of the fermentation, diameter and depth of batter deposits in the automatic Idli manufacturing unit developed at the authors＇ laboratory were measured. Whereas the depth remains almost same for the batter types, there was a decrease in diameter as the ratio of rice to black gram increases. (C) 2000