Rat is a rodent (rodensia) which cannot be separated from parasitic organism attacks the ectoparasites(fleas). In the presence of fleas plague focus areas need to watch out, for no increase in cases of plague(outbreak). Pest is a zoonosis in rat that can be transmitted to humans through the bite of fleas Xenopsyllacheopsis containing Yersinia pestis. Boyolali District is one of the plague focus areas in Central Java. Thisstudy aims to identify the species of rats and fleas, trap succes, flea infestation in rats and flea index as anindicator of vulnerability to transmission of plague. The study is a descriptive survey with cross sectionaldesign. The population is all the rats and fleas in Boyolali district. Samples are rats and fleas that werecaught using live trap with coconut roasted and salted fish is placed inside and outside the home (each 2trap). Rat combed for fleas. The results showed the number of mouses caught were 245. There are 4 speciesrats and small mammals found in R. tanezumi, R. tiomanicus, R. exulans, N. fulvescens and S.murinus withsucces trap at 5.71%. Only 3 species and S.murinus of infected fleas. Species of flea is X. cheopis and S.cognatus. Specific flea index: Xenopsylla cheopis by 1.67; flea index cognatus Stavilus common flea indexof 0.88 and 2.55. Based on the warning system indicator about the bubonic plague spreading,which isspecific flea index of X.cheopis >1 and fleas index >2, Selo sub distric should be aware to the spreading ofbubonic plague in its area, so that it is important to carry out the controlling of rat and flea population.