The influence of temperature and relative humidity in infection and cross-infection of Nosema bombycis and N. mylitta respectively in mulberry silkworm, Bombyx mori L. on larval mortality, multiplication of pathogens, larval weight and growth rate in three different seasons were studied. Seasons were selected in such condition, when very less fluctuations between minimum and maximum temperature and minimum and maximum relative humidity ($25{sim}28^{circ}C$ and $65{sim}72%$ R.H) was observed i.e., season-1. Fluctuations between minimum and maximum temperature were less ($28.05{sim}34.50^{circ}C$) but R.H % was more ($55{sim}81%$) in season-2. Fluctuations between minimum and maximum temperature and R.H % were more ($20.00{sim}40.5^{circ}C$ and $64.00{sim}90.00%$) in season-3. Growth rate of microsporidian-infected silkworm is directly related to the prevailing temperature and relative humidity in silkworm. Silkworm can tolerate slight variation of temperature but slight variation of relative humidity disfavours the development of silkworm and favours the multiplication of pathogens.