The quality of the groundwater along the slopes of Mount Meru, Tanzania, is poorly understood. Water access and sanitation practices may pose health risks to communities. This study was undertaken to assess the sources, abundance and seasonal variation of coliform bacteria in groundwater and factors contributing to such variations along slopes of Mount Meru, Tanzania. Water samples collected from 67 randomly selected water sources (springs, shallow wells which ranged from 4 to 35 m deep and Boreholes above 40 m deep) during dry and wet seasons were analysed for total coliform (TC), faecal coliform (FC), Escherichia coli (E. coli) and faecal streptococci (FS), using the membrane filtration method. The fraction of springs and shallow wells contaminated was generally higher compared to the fraction of boreholes. The highest TC, FC, E. coli and FS counts were significantly higher (p?<?0.05) during the wet than the dry season owing to rising of water table and leaching during rainy season. Water sources that were located within 10 m of pit latrines had the highest coliform counts relative to those located beyond 10 m. Similarly, the highest coliform counts were observed in all shallow wells that (i) had low well head above the ground, (ii) were not covered, (iii) had casing materials which were not concrete and (iv) utilised traditional pumping (bucket/pulley) systems. This was due to contaminated storm water access, inoculation of microbes by exposed buckets and inefficiency of the casing material. Furthermore, the counts decreased with depths of boreholes and shallow wells during the two seasons probably due to retention and die-off. It is recommended that groundwater in this area be treated against coliform contamination prior to utilisation as portable water.