This will be a double blind comparative study, performed in pediatric subjects (Age 12-17) traveling to developing regions with a known high incidence of traveler's diarrhea. The subjects will be suffering from acute diarrhea for at least 12 hours, without symptoms of systemic infection.

Approximately 142 subjects are expected to be enrolled in the study, 1:1 in the Rifamycin SV MultiMatrix (MMX) plus Oral RehydrationTherapy (ORT) group and in the placebo tablets plus ORT group respectively. The day of randomization (Visit 1, Day 1), the subjects will start the treatment receiving ORT plus Rifamycin SV MMX 400 mg (as two tablets of 200 mg each) twice daily (morning and evening) or ORT plus placebo tablets (as two tablets) twice daily (morning and evening). The subjects will begin the treatment within 72 hours of onset of diarrhea. Treatment duration will last 72 hours. The total number of tablets for the entire treatment course will be 12 (4 × 200 mg tablets/day for 3 days). The administration of the ORT will follow the specification reported in the product label. The tablets will be orally administered during the day. No tablet administration will be done during the night.

After enrollment, subjects/their parents/or guardians will complete Diary Cards (PDC) in which will be daily recorded date, time of first tablets intake, time and consistency of each stool, presence of blood in stools, abdominal pain/cramps, flatulence, tenesmus, urgency, nausea, vomiting, fever, adverse events (AEs), and concomitant medications.

During the study, the subjects will be assessed for safety and efficacy at Visit 2 (Day 2) and Visit 3 (Day 4/5) as final Study Visit.

Stool samples for microbiological assessment will be collected at Visits 1 and 3. Stool will be examined and cultured at local labs for main enteropathogens and for the presence of protozoa, ova and yeasts.

Stool samples will be required in a subset population of the Rifamycin SV MMX group, at the Follow-up visit to determine rifamycin concentration in the feces.

Blood and urine sampling for routine safety analyses will be collected at Visit 1 and at Visit 3.

Inclusion Criteria:

• Diagnosis of acute bacterial diarrhea defined as at least 3 unformed stools within the 24 hours preceding randomization, with a duration of illness ≤72 h. The bacterial origin of diarrhea will be confirmed "a posteriori" by stool microbiology sampling at the time of screening.
• Presence of one or more signs or symptoms of enteric infection have to be present, including nausea, vomiting, abdominal cramps or pain, tenesmus, urgency
• History of recent travel from an industrialized country to a developing region with a known high incidence of travelers' diarrhea
• Male or female 12-17 years of age, providing an unformed pre-treatment stool
• Females of child-bearing potential must use an acceptable contraceptive method throughout the study treatment period
• The parent or legally acceptable representative guardian must provide informed consent for the subject. The Subject must also provide written informed assent by the parent or legal guardian at the time of assent/consent signing.

Exclusion Criteria:

• Fever (>100.4ºF or 38ºC), or presence of signs and symptoms of systemic infection
• Females pregnant or breast feeding or not using adequate birth control
• Known or suspected infection with non-bacterial pathogen
• Symptoms of acute diarrhea of >72 hours duration
• Presence of grossly bloody stool
• Moderate to severe dehydration
• History of inflammatory bowel disease (IBD)
• Abdominal ileus
• Severe dehydration
• Greater than two doses of an antidiarrheal medication within 24 hours before randomization, or any symptomatic therapy within 2 hours before enrolment
• Receiving antimicrobial drug with expected activity against enteric bacterial pathogens within the week prior to enrolment
• Hypersensitivity to rifamycin-related antibiotics or to any excipient included in the study medications
• Subjects unable/unwilling to comply with study protocol
• Participation in a clinical trial within the last 30 days