During the spring-summer 2010 and fall-winter 2011 crop cycles, an intensive search was done for fall armyworm larvae, Spodoptera frugiperda (J.E. Smith), in maize (Zea mays L), in the area of Guasave, Sinaloa, Mexico. Larvae with signs of viral infection were chosen for DNA extraction using the method of DNAzol for detection of virus by PCR. Diagnosis of virus was done using a 575-bp fragment of the gene isolation polyhedron from the native baculovirus SfMNPV. Third-instar larvae that tested positive for the virus were inoculum for infectivity bioassays, and the infective dose, median lethal dose, and lethal time were calculated. The prevalence of SfMNPV was 0.7% in autumn-winter 2010 and 2.1% in spring-summer 2011. Infected larvae were found at 43% of the sampling sites throughout an area 20 km² The PCR product (575 bp) of SfMNPV native virus isolate was sequenced (Genbank acc. No. KC845532) and had 99% identity to the SfMNPV polyhedrin gene (Genbank acc. No. J04333.1). The SfMNPV virus infectious titer had an average rate of 10^4.59 ± 10^0.09 DI₅₀/mL and TL₅₀ of 111.56 ± 0.46 hours. By 132 hours post inoculation all infected insects died, showing previous signs of lethargy and anorexia, however the larvae stopped feeding at 84 hours. These results indicated that the virus SfMNPV detected in the surveyed area was an infective agent and potential biological control agent for fall armyworm larvae, so it is necessary to assess virulence in the field.