A field study (1998-99) was conducted at the Juana Díaz Substation (AESUPR) to determine the population dynamics of Plutella xylostella (L.) in cabbage in a monoculture and in relay-type substitutive dicultures of cabbage/tomato or cabbage/wedelia. In 1998, subplots were created by spraying half of the main plots with Bacillus thuringiensis (Bt)-based products. All treatments, except for the control monoculture, were sprayed with Bt-based products in 1999. Cotesia plutellae Kurjumov, a larval parasitoid of P. xylostella, was released augmentatively for biological control. Tomato, as a companion crop, showed a tendency for reducing P. xylostella densities in cabbage, reduced the levels of C. plutellae parasitism and competed with cabbage, causing fewer and smaller heads. Bt-based sprays reduced P. xylostella densities, but these were at damaging levels at the critical stages of cupping and head formation, thus permitting cosmetic damage to cabbage heads. Cotesia plutellae did not regulate the P. xylostella population, thus resulting in parasitism levels of 65.3 and 11.6% in the unsprayed monoculture in 1998 and 1999, respectively. The legume Crotalaria juncea L. as a border did not improve parasitism by C. plutellae. The interpretation and application of the results are discussed.