Abstract. Arsita EV, Annisa. 2019. Water cytotoxicity assessment of Rajamantri River, Pananjung Pangandaran Nature Reserve, West Java using Allium cepa as bioindicator. Pros Sem Nas Masy Biodiv Indon 5: 95-100. Water is the main necessity that needed by all organisms. Toxic substances in water can cause pollution and give some effects to organisms that depend on the water source. Cytotoxicity test using Allium cepa L. (onion) as a biological indicator is one of an effective way to detect the quality of water. This research was conducted to detect the level of cytotoxicity based on mitotic index and chromosome aberration in the onions roots cell. Observation and completely randomized design was used for this research. Onionswere planted firstly in water sample which was taken from three points in Rajamantri River i.e.: an upstream, middle, and downstream. Squashing technique was used to make the preparations of onions root cells. Mitotic index (MI) was analyzed statistically using analysis of variance (ANOVA) and then continued with the Tukey post-hoc test. The types of aberration were described descriptively. The MI values decreased from upstream (64.3%), middle (53.92%), and downstream (51.52%), with control MI is 79.23%.Types of aberration observed as many as 13 types, which include bridge, sticky, and c-mitosis. The difference value of MI between the middle and the downstream with the control showed the water toxicity level in these part of river was categorized sublethal on the onions root cells.