Nosema sp. infection in the Indian tasar silkworm, Antheraea mylitta exerts a complex of influences on its host. The instar duration was extended significantly (P<0.001) except in $1^{st}$ instar. The infected larvae took about 48 days to reach the spinning stage against 40 days in the uninfected ones. The final weight attained by the larva at the end of each instar of development declined significantly following infection, as did weight gain and relative growth rate (RGR). The growth recorded/ day declined in infected larvae compared to uninfected ones from 8.2% during $1^{st}$ instar to 43.3% during $5^{th}$ instar. Food ingestion and digestion increased with advancement of the instar significantly irrespective of the status of the larvae but the relative consumption rate (RCR) declined. These parameters significantly declined in infected larvae (except food digested during $2^{nd}$ instar). The decline was more during $3^{rd}$ instar. In contrast, the approximate digestibility (AD %) was significantly higher in infected larvae than uninfected ones leaving the $1^{st}$ instar larvae unaffected. The efficiency of conversion of ingested food (ECI) and efficiency of conversion of digested food (ECD) did not change in a patterned way following the microsporidia (Nosema sp.) infection. The values of ECI significantly changed during $2^{nd}$, $3^{rd}$ and $5^{th}$ instars; while the change in ECD during $2^{nd}$, $4^{th}$ and $5^{th}$ were significant. During the entire larval life all the parameters tends to decline significantly following microsporidia infection but AD registered a significant increase. Nosema sp. spore concentration has increased 270.7 times during larval development in the course of experimentation.