A survey of different weedy rice biotypes was conducted in rice field areas of Nueva Ecija from CY 2004-2006 to determine their prevalence, agronomic characteristics and biology. Surveys and quadrat samplings were conducted in random alongside of the roads to gather data on prevalence, height, number of panicles/plant and density/m2 of weedy rice as well as the cultivated ones they were associated. Studies on seed germination test, biology and agronomical characteristics of different weedy rice biotypes were also conducted at the laboratory and screenhouse of Crop Protection Division, Philippine Rice Research Institute. Five weedy rice biotypes designated as WR-NE 1, WR-NE 2, WR-NE 3, WR-NE 4 and WR-NE 5 were found distributed almost everywhere in rice fields of Nueva Ecija. Among the biotypes, WR-NE 1 was the most common in many rice fields. Prevalence of all biotypes around the province ranged from 1 to 17%. In the field, all biotypes were found taller but had fewer panicles plant-1 than cultivated rice varieties. In the laboratory, all weedy rice biotypes emerged earlier than PSB Rc82 and IR64. Under screenhouse condition, all weedy rice biotypes were taller, produced more leaves and culms plant-1 than the cultivated rice. WR-NE 5 was the tallest biotype while WR-NE 1 and WR-NE 2 had the most number of leaves and culms plant-1. All biotypes matured earlier but had no seed shattering characteristics. The biotypes had comparable values in terms of flag leaf areas and LAI/plant with PSB Rc82. Shoot dry weight and number of panicles/plant of all biotypes was significantly higher than the control varieties. WR-NE 4 and WR-NE 5 had the longest panicles, longer and wider grains. Grains/panicle was highest in WR-NE 5 (102 grains/panicle). Grain length width ratio (GLWR) of all biotypes, however, were significantly lower than the two control varieties. WR-NE 1 and WR-NE 4 had the longest length of awn while WR-NE 4 and WR-NE 5 had the highest grain weight (both at 24 g/1000- grains). The growth and development of the five weedy rice biotypes were much advantageous compared to selected cultivars suggesting their competitiveness and threat in achieving the potential yield and quality of harvested cultivated rice.