The in vitro susceptibility response of Staphylococcus aureus, Klebsiella pneumoniae, Escherichia coli, Proteus mirabilis and Pseudomonas aeruginosa to a set of antibiotics was investigated in a survey comprising 19,380 positive cultures over a period of 5 years in a large hospital environment. Four out of the five species (P. aeruginosa being the exception) presented a species-specific, drug-independent, rhythmic variation of their level of susceptibility to several antibiotics over the time of the study. The species-specific rhythmic responses were further characterized by spectral analysis, autocorrelation and cross-correlation functions. Through this analysis it was possible to rank the species according to their main period of oscillation. The longest period of oscillation was detected for S. aureus (38 months). K. pneumoniae and E. coli presented intermediate values (25 and 23 months respectively), and P. mirabilis the shortest period of oscillation (11 months). Species displaying long periods of oscillation tended to present very low levels of susceptibility, while species displaying short periods of oscillation usually presented the highest levels of susceptibility observed. Although some hospital environmental factors, such as drug consumption, were also analyzed, no correlation was found between them and the in vitro bacterial cyclic responses to antibiotics.