The maternity colony of Myotis formosus in Yun-lin County, southwestern Taiwan, began to form in late March, and the numbers peaked (about 200) in May. Most bats left the roost site between mid-August and early October for unknown hibernation sites. Females gave birth during May and June. Before parturition, females (n = 19) spent 563.9 min ± 47.28 SD outside the roost per night. Time away from the roost decreased significantly after they gave birth. Lactating females increased the total time that they spent outside the roost per night by 37.8% from early to late lactation. Females spent more time grooming young than self-grooming during the first 2 weeks of lactation. From the 3rd week on, more time was spent on self-grooming. Forearm length and body mass of newborns were 21.46 ± 1.90 mm (n = 13) and 3.71 ± 0.41 g (n = 12), about 42.8% and 24.1% of adult size, respectively. Young rarely moved during the 1st week after birth. They started to crawl at the 2nd week and to attempt short straight-line flight at the 3rd week. Females always rested next to their young while roosting in the first 2 weeks of lactation but separated from them from the 3rd week on. Behavioral changes in females coincided with the growth and development of their young.