Monoxenous trypanossomatids protozoa are not believed to cause in vivo infection in vertebrate hosts throughout their life cycle. However, there are reports mentioning some cases of HIVpositive patients who have presented opportunistic infections caused by these protozoa. Recently, we have demonstrated the in vitro infection of mouse dermal fibroblasts by these protozoa. The aim of the present work is to investigate the possibility of Crithidia deanei, a endosymbiont-bearing monoxenous trypanossomatid, infect BALB/c mice under or not Dexamethasone treatment. To attend it, distinct gro- ups of adult BALB/c mice were immunosuppressed with 50 mg/kg of Dexamethasone. This immunosuppressor was administered 24 hours before infection and daily, for 15 days after C. deanei inoculation. Control groups: C. deanei–inoculated animals but non-immunosuppressed and non-inoculated animals but immunosuppressed were also used. Light Microscopy analysis revealed an infection process characterized by the presence of the trypanossomatid inside dermal cells in the groups studied. The experimental inoculation resulted in a non-lethal infection characterized by the presence of the trypanossomatid inside dermal cells in the normal BALB/c mice, but notably, in the C. deanei–inoculated immunosuppressed group. These preliminary results lead to the following conclusions: 1) C. deanei is able to infect normal BALB/c mice; 2) the immunosupressed mice seemed to be more susceptible to the C. deanei infection compared to the control group. Besides C. deanei in dexamethasone-immunosuppressed mice provides a useful model for studies of monoxenous trypanosomatids ‘in vivo’ infection, resembling that one presumably occurring in imunodeficient individuals with AIDS.