A total of 164 isolations of Serratia marcescens achieved during 1978-1979 at the Hospital Infantil de México in children with various pathology due to this bacteria were studied. Most of the cases were debilitated patients from the newborns and prematures wards and contagious and surgery departments. The most frequent isolations were from wounds and abscesses (76 cases), the same as from meningitis (22 cases) and sepsis (12 patients). Serratia marcescens showed a high degree of resistance (87-100%) to the following antibiotics: carbenicillin, colimycin, chloramphenicol, phosphomicin, ampicillin and cephalothin. To gestamicin and kanamycin, 42% of strains were sensitive. Amikacin was the most effective drug with 92% of strains susceptible to it. The history of this bacteria, its mode of transmission, frequency of infections and resistance to antibiotics found in foreign institutions are commented. Likewise, the difficulty for the precision bacteriologic diagnosis is emphasized as the possible main cause for the ignorance in Mexico of infections due to this bacteria.