Bacillus thuringiensis (Bt) strain K4 was isolated from fallen leaves which had been collected at a forest stand in Mungyeong city, Republic of Korea. The flagellated vegetative cells of Bt K4 were agglutinated with the H3 reference antiserum among 55 reference H-antisera. In a further test to identify subfactors, 3b and 3d monospecific antisera were reactive to the cells, followed up with introducing a novel serogroup of 3a3b3d, designated as serovar mogi. The strain K4 had mosquitocidal activity against Dipteran larvae, Anopheles sinensis and Culex pipiens pallens, with no Lepidopteran toxicity observed. The SDS-PAGE profile of K4 crystal protein, ovoidal-shaped, included several bands ranging from 30-75 kDa. Four putative peptides, Cry19Ba, Cry40ORF2, Cry27Aa and Cry20Aa were detected from the bands by a nano-LC-ESI-IT MS analysis. Through a thermal asymmetric interlaced PCR, cry19Ba, cry40ORF2 and cry27Aa genes were partially cloned from K4 strain. Three cry genes were further found in the strain by a 454 pyrosequencing, ending up to showing 58%, 39% and 84% homology in amino acids with Cry56Aa, Cry8Ba and Cry39ORF2 toxins, respectively. This novel 3a3b3d type strain, B. thuringiensis subsp. mogi, can be used as a good resource for studying unknown mosquitocidal cry genes.