Distinctive within the North American fauna in having the following combination of characteristics: small (3.5–4.2 mm in length), metallic coloration, with surface dull because of well-impressed, isodiametric sculpticells (Fig. 1). Pronotum slightly constricted posteriorly (Fig. 2); posterior margin slightly narrower than anterior margin. Elytron with humeral margin forming a sharp angle (Fig. 2), with lateral groove reaching to near the base of the fourth stria; two discal setae contained in small foveae. In addition, the anterior supraorbital seta is in a furrow that extends back toward the posterior supraorbital setae (Fig. 3); this furrow is separated from the central portion of the head by a carina. The internal sclerites of the male genitalia are also distinctive within the North American fauna (Fig. 4).

A Mediterranean species, from Spain and Portugal east to Israel and Iraq, on various Mediterranean islands, and in northern Africa from Morocco to Algeria, Tunisia, and Libya, and the Azores (Matallah et al. 2016, Ortuño and Toribio 2005, Marggi et al. 2017). In North America, it is has been found at five localities in the San Francisco Bay, California region (Fig. 5).

The records from California, from the earliest to latest records, are as follows:

• USA: California, Santa Clara Co., Palo Alto, Stanford University Campus, 37.4224°N 122.18124°W. 12.xi.2012. Lindgren funnel trap, Sirex lure, collector R. Sloan. (1 female, JRLC.)
• USA: California, Santa Clara Co., Santa Clara, Forge Garden and Santa Clara University, 37.35229°N 121.93941°W. 20.v.2013. (1 male, EMEC346362/1 female EMEC346363.)
• USA: California, Santa Clara Co., Gilroy, Thousand Trails, 4.xi.2015. Lindgren funnel trap with Quercivorol, collector R. Sloan. (1 female, JRLC.)
• USA: California, Santa Clara Co., San Jose, 1890 Dobbin Drive, 21.xii.2016. Lindgren funnel trap with Manuka oil, collector C. Higgenbottom. (1 male, JRLC.)
• USA: California: Alameda Co., Oakland, 37.8120°N 122.2717°W. 23.iv.2017, 19.xi.2017, and 2.xii.2017. Collector S. Crews. (1 male, EMEC346364/1 female EMEC346365; 1 male, OSAC DRMaddison DNA Voucher DNA5191.)

In Oakland, California, the beetles were found in a backyard in an urban environment with non-native grasses and a mixture of native and non-native shrubs. Collections were made during the day, and the specimens were found in leaf litter around a small coffeeberry (Frangula californica), under a birdbath on moist soil, and at the base of dandelions (Taraxacum officinale).

The key of Lindroth (1963) to species groups of Bembidion can be modified as follows to accommodate the B. ambiguum species group (page 209):

7. 8. elytral stria removed from lateral bead at least half as much as from 7. stria ................. 8

– 8. stria closer to side-margin ................. 7a

7a. Anterior supraorbital seta at the front end of a deep furrow; this furrow is separated from the center of the head by a carina ................. ambiguum group

– Anterior supraorbital setae not in furrow, without carina mesal to this ................. 10

The key of Lindroth (1963) to species of Bembidion can be similarly modified as follows to accommodate B. ambiguum (page 213):

20. 8. elytral stria removed from lateral bead (at middle) at least half as much as from 7. stria. (All striae evident to apex) ................. 21

– 8. stria much closer to side-margin ................. 20a

20a. Anterior supraorbital seta at the front end of a deep furrow. Pronotum slightly constricted at base. Metallic coloration; with strong isodiametric microsculpture over the entire dorsal surface. Elytra with discal setae in shallow foveae. 3.5–4.2 mm in length ................. B. ambiguum

– Anterior supraorbital seta not in deep furrow. Remaining characters not as above ................. 31