We aimed to determine seed germination responses and flood tolerance of Ludwigia hyssopifolia and L. perennis that grow in rice fields in Rajgir, India. Freshly-matured seeds were incubated in 12 hr / 12 hr light / dark and complete darkness at constant 25 oC and natural daily fluctuating temperatures. Seeds exposed to different light durations were then incubated in complete darkness. Seeds exposed to different flooding durations were incubated in continuous flooded or non-flooded environments. Seeds of both species germinated within four days in light/dark but failed to germinate in complete darkness, revealing their nondormant and positive photoblastic behavior. Some seeds of both species (10 – 20 %) germinated in complete darkness after exposure to light for 24h. Seeds failed to produce normal seedlings in a continuously flooded environment. Seeds of the two studied species tolerate at least one week of flood. Seeds of L. perennis have a higher tolerance to flooding than those of L. hyssopifolia, which survived four weeks in a flooded environment. The two species have the same germination behaviour but differ in ability to tolerate flooding. Since seeds of both species are nondormant, positively photoblastic, and have different degrees of flood tolerance, a flooding regime of rice fields will not be sufficient to control these weeds.