The taxonomic portion of this paper discusses 27 species of mites of the family Phytoseiidae known to occur on plants in Madagascar. Eight new species are described: Phytoseius (Phytoseius) betsiboka. Ph. (Pennaseius) onilahy, Amblyseius (Amblyseius) hova, A. (A.) ankaratrae, A. (A.) boina, A. (A.) sakalava, A. (A.) trichophilus and A. (Proprioseiopsis) tulearensis. The following species are recorded for the first time from the island: Typhlodromus (Typhloseiopsis) contiguus Chant, 1959, Phyloseius (Phytoseius) crinitus Swirski & Shechter, 1961, Ph. (Ph.) intermedius Evans & Macfarlane, 1961, Ph. (Pennaseius) hongkongensis Swirski & Shechter, 1961, Ph. (P.) amba Pritchard & Baker, 1962, Iphiseius (Iphiseius) degenerans (Berlese, 1889), I. (Trochoseius) gongylus Pritchard & Baker, 1962, Paraphytoseius multidentatus Swirski & Shechter, 1961, Amblyseius (Amblyseius) hima Pritchard & Baker, 1962, A. (A.) deleoni Muma & Denmark, 1970, A. (Proprioseiopsis) sundi Pritchard & Baker, 1962 and A. (P.) peltatus Van der Merwe, 1968. Additional descriptions are given for most of these species. The male of P. multidentatus is described for the first time. New distribution records are presented for the following species: Typhlodromus scytinus Chazeau, 1970, the T. (Anthoseius) gutierrezi-chazeaui group, Amblyseius (Amblyseius) masiaka Blommers & Chazeau, 1974, A. (A.) bibens Blommers, 1973, A. (A.) rotundus Blommers, 1973, A. (A.) brevipes Blommers, 1973 and A. (A.) tamatavensis Blommers, 1974. The simultaneous occurrence of spider mites and other small arthropods on the plants on which the phytoseiids were collected is noted. A final chapter summarizes the results of experiments devised to see whether the collected phytoseiid mites can be effectively predators of spider mites, i.e. of Tetranychus neocaledonicus André, 1933 and sometimes Oligonychus coffeae (Nietner, 1861). Most of these results were negative. Only, Typhlodromus contiguus, Amblyseius tulearensis and A. sundi can regulate population numbers of the first prey species under circumstances of low webbing and/or additional food. Iphiseius degenerans, and possibly Amblyseius rotundus, can control Oligonychus coffeae in their area of distribution.