The microscopic morphology, that is, total hyphal length and total number of tips, has been characterized during batch cultivations of Aspergillus oryzae. The specific growth rate estimated by measuring the total hyphal length (mu(h)) corresponds well with the specific growth rate estimated from dry weight measurements during cultures grown as free hyphal elements. The average tip extension rate can be described with a saturation type kinetics with respect to the average total hyphal length, and the branching frequency is closely related to the total hyphal length. For the applied strain of A. oryzae, pellet formation occurs by coagulation of spores. The agglomeration process is pH dependent and pellets are formed at pH values higher than 5, whereas low pH (<3.5) results in growth as freely dispersed hyphal elements. The maximum specific growth rate has a broad pH optimum between 3 and 7, whereas the alpha-amylase production has a sharper maximum at about pH 6. During batch cultivation with pellets the growth is described well by the cube-root law when pellet fragmentation can be neglected. The kinetic parameter k in the cube-root law is derived from the growth kinetics with no mass transfer limitation, k = mu(h)/3 Based on an oxygen balance, the active growth layer in the pellet is estimated to be 200 to 325 mu m and, consequently, up to 50% of the biomass is limited by oxygen for large pellets. Ethanol production (up to 1 g L(-1)) was observed during batch cultivations with pellets, suggesting that ethanol is produced in the oxygen limited part of the biomass. A constitutive, low alpha-amylase production was observed at high glucose concentration. The specific alpha-amylase production was significantly higher for filamentous growth than for pellets and oxygen appears to be necessary for production of alpha-amylase.