This study was carried out to evaluate the effects of in-service-teacher-developmentprogramme on pupils’ interest in lower basic mathematics in Katsina state, Nigeria. The study adopted sample cross sectional survey design. The sample for the study was 700 pupils comprising 358 and 342 male and female pupils selected from a population of 45,746 primary five pupils from six Local Government Education Authorities (LGEAs) in Katsina Education Zone – three LGEAs were in the programme (ITDP) involving 361 pupils and the other three were not (Non-ITDP) with 312 pupils. Data collection was accomplished through the use of Primary Mathematics Interest Inventory (PMII). The PMII consisted of 40 items and was adapted from Haun (2016). The instrument was validated by experts and reliability coefficient was computed and found to be 0.78 Mann-Whitney U test was used to analyse the data. Findings showed that pupils from ITDP schools exhibited positive` effect on interest in comparison to their counterparts from Non-ITDP schools and the effect of the intervention was gender neutral on interest within ITDP schools. Based on the findings of the study, it was recommended that Katsina state government need to consider replicating the programme in LGEAs where it was not conducted and professional bodies popularise strategies of the programme through series of workshops and conferences.