The social amoeba Dictyostelium discoideum is a promising host for the expression of recombinant proteins requiring post-translational modifications. Limited maximal cell densities and slow growth rates, however, disfavor its application. Little attention has been paid to improve its cultivation. Here, some strategies are described, which allow the attainment of higher cell densities. This can be achieved by cultivation on an improved synthetic medium as well as in immobilized form. Two promising inorganic porous supports are presented-broken pumice and a ceramic catalyst carrier. Cell densities up to 4.5 x 10(7) ml(-1) are obtained during suspension cultivation on the improved synthetic medium. This is about three times as much as can be expected for cultivations on conventional complex media. Cells in the pores of broken pumice and a ceramic support reach up to 15-20 times higher local cell densities compared with cells growing on conventional axenic media in suspension. Thus, the cell density of 3.5 x 10(8) ml(-1) in the ceramic carrier (CeramTec(R)) is the highest cell density for D. discoideum observed so far in cultivation systems. The immobilized cell density could be maintained for a long period of time by either repeated medium replacement or continuous cultivation. To minimize the external volume of the medium a trickle bed reactor was operated continuously. (C) 2003 Elsevier Ltd. All rights reserved.