Abstract 1. 1. The transport of a marker organic acid, fluorescein in isolated Malpighian tubules of the tropical cockroach ( Blaberus giganteus) and American cockroach ( Periplaneta americana), Asiatic locust ( Locusta migratoria) and larva of the wax moth ( Galleria mellonella) has been studied quantitatively by means of a microfluorimetric method. 2. 2. Fluorescein transport in the Malpighian tubules of all species occurred against the concentration gradient, followed saturation kinetics and was inhibited by other carboxylic acids ( p-aminohippurate, probenecid). Fluorescein transport in the Malpighian tubules of B. giganteus was also inhibited by sulphonates (indigocarmine, congo red). 3. 3. Active fluorescein transport in the Malpighian tubules of all species under study was shown to depend on the temperature of the incubating medium. 4. 4. The values of the Michaelis-Menten equation parameters for fluorescein transport in Malpighian tubules: the apparent Michaelis constant ( K M), inhibition constant ( K i) and maximal transport rate ( V max) were determined at the optimal temperature for each species. Alterations of K M and V max indicate that fluorescein-transport inhibition by carboxylic acids ( p-aminohippurate, probenecid) is competitive while its inhibition by sulphonate, congo red, in the Malpighian tubules of B. giganteus is mixed. 5. 5. The transport of fluorescein is sodium-dependent in the tubules of all species. 6. 6. The active transport of organic acids in the Malpighian tubules of insects is, in many respects, similar to that in the renal proximal tubules of vertebrates. 7. 7. Correlations are revealed between the value of V max K M ratio and the type of nutrition and some peculiarities in the ecology of the insects in question.