A new temperature-sensitive mutant strain of Escherichia coli K-12 which forms filaments at 42 C has been described. The mutant, Y16, maintained growth and deoxyribonucleic acid synthesis at 42 C. The resulting multinucleate filaments gradually lost their viability at 42 C but could be recovered, even after 240 min of incubation, upon return to 30 C. Septation was resumed and growth was promptly re-established at normal rates. Recovery still took place in the presence of chloramphenicol added to the culture at the time of temperature shift from 42 to 30 C. A study has been made of the effects of adenine and various nucleosides on cultures of strain Y16 as compared with another filament-forming mutant, T44 tif-. Adenine (75 mug/ml), known to promote filamentation of strain T44 tif-, prevented the development of filaments and the loss of viability in cultures of Y16. Recovery of septation after temperature shift in cultures containing adenine presented a pattern similar to that found with the adenine-less cultures. Protection afforded by adenine at 42 C could be reversed by the addition of guanosine plus cytidine (100 mug/ml each). The effects of high concentrations of adenine and nucleosides on strain Y16 thus are the reverse of those observed with mutant T44 tif-. However, whereas tif-1 mutation promotes prophage induction at restrictive temperatures, no modification could be detected in the process of prophage induction in cultures of the lambda-lysogenic derivative of Y16 at 42 C, be it spontaneous or ultraviolet-mediated induction. The osmolarity increase afforded by 1% NaCl added to the medium did not alter the phenotype characteristics of strain Y16. The mutation has been mapped between argG and bgl. A close linkage has been observed between ftsH and argG, thereby locating the new mutation near 61 min on the map of E. coli chromosome, a previously undescribed region involved in cell division. The evidence reported indicates that strain Y16 differs in several respects from the already descirbed strains of the same class.