This study was conducted aiming to evaluate interactions between deleterious action of biofertilizers on Brevipalpus phoenicis (Geijskes) (Acari: Tenuipalpidae) and host plants. In the first bioassay the following concentrations were tested: 0% (control), 5%, 15%, 30% and 50%. The experimental unit was a leaf of Ligustrum lucidum plant (six leaves per treatment). Twelve mites were liberated in arenas on each leaf. The second bioassay was run on 20 days old Canavalia ensiformis plants, using ten plants per treatment. Ten mites were transferred for arenas on each host plant cotyledonal leaf. Five concentrations were also tested: 0%, 5%; 10%; 20% and 30%. The adult female mortality and the number of eggs laid in each arena were quantified daily during 72 h in the first assay and 120 h in the second one. The mite survivorship and oviposition were significantly reduced with the increase of biofertilizer concentration in both experiments. The LC25 estimates for the 24, 48 and 72 h periods were respectively 50,04; 15,70 and 4,95% in L. lucidum. The LC25 andLC50 estimates for the 24, 48, 72,96 and 120 h periods in C. ensiformis were 8,15; 7,78; 0,63; 0,68 and 0,63 and of 19,64; 19,03; 2,38; 2,60 and 2,67, respectively. No LC50estimates were obtained in L. lucidum, due the low mortality rates. The biofertilizer had deleterious action on fertility and survival of B. phoenicison both host plants, being more severe on C. ensiformis. The mites dead by the biofertilizer action showed evidences of microbial colonization. A colloidal compound of the biofertilizer induced mite immobilization and obstruction in its digestive tract.