In this paper, we explore the understudied phenomenon of "low-frequency" heroin injection in a sample of street-recruited heroin injectors not in drug treatment. We conducted a cross-sectional study of 2,410 active injection drug users (IDUs) recruited in San Francisco, California from 2000 to 2005. We compare the sociodemographic characteristics and injection risk behaviors of low-frequency heroin injectors (low-FHI; one to 10 self-reported heroin injections in the past 30 days) to high-frequency heroin injectors (high-FHI; 30 or more self-reported heroin injections in the past 30 days). Fifteen percent of the sample met criteria for low-FHI. African American race, men who have sex with men (MSM) behavior, and injection and noninjection methamphetamine use were independently associated with low-FHI. Compared to high-FHI, low-FHI were less likely to report syringe sharing and nonfatal heroin overdose. A small but significant proportion of heroin injectors inject heroin 10 or less times per month. Additional research is needed to qualitatively examine low-frequency heroin injection and its relationship to drug use trajectories.